Forty years ago, the Mets did the unthinkable. They traded Tom Seaver — The Franchise — to the Reds in a deal that many longtime fans have neither forgotten nor forgiven.

The trade deadline of June 15, 1977, became a day of wreckoning as the Mets sent their future Hall of Famer packing over a salary dispute with board chairman M. Donald Grant.

“What were they thinking?’’ former teammate Bud Harrelson said recently, echoing a question that Mets fans have been asking since the deal.

Seaver’s departure was a seismic event that lit up the Shea Stadium switchboard with calls from angry fans shaken by the trade of the team’s star, who had won three Cy Young Awards.

With the beginning of big free-agent contracts, Seaver, in spring training in 1977, wanted to renegotiate the three-year deal he signed in February 1976 that paid him a base salary of $225,000 a year.

Before signing that deal, Seaver had told then-team president Lorinda de Roulet, daughter of founding owner Joan Whitney Payson, that the time had arrived for the $200,000-plus price tag

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“She said, ‘Over my dead body,’ ’’ Whitney de Roulet Bullock, the owner’s daughter, said from her home in upstate Millbrook.

Grant, who ran the baseball operations, would not consider renegotiation. Seaver was shipped to Cincinnati for top minor league outfield prospect Steve Henderson, pitcher Pat Zachry, utility infielder Doug Flynn and minor league outfielder Dan Norman.

Harrelson recently visited Seaver and his wife, Nancy, at their Napa Valley vineyard in Calistoga, California. Seaver has been coping with some effects of Lyme disease, a bacterial infection transmitted by ticks that he traced to when he lived in Greenwich, Connecticut, in 1991. “The Lyme has impacted his short-term memory, but he is in great physical health,” said Seaver’s daughter, Anne.

Harrelson said Seaver “looks good, he’s happy.’’

Seaver, 72, declined to be interviewed.

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Harrelson, 73, said he recalled that evening in Atlanta when Seaver was traded. “I cried,’’ Harrelson said. “He was my buddy. We were roomies. I went and hugged him and cried.’’

Seaver, coming off a season in which he went 14-11 with a 2.59 ERA, was 7-3, 3.00 at the time of the trade. He had extricated the Mets from their early years as baseball’s bumblers and led them from 1968’s ninth-place finish to the 1969 World Series championship.

Lorinda de Roulet was appointed team president by her father, Charles Payson, who took over ownership of the Mets when Joan Payson died. The Harvard-educated Payson identified himself as a Red Sox fan. Grant had been Mrs. Payson’s financial adviser. Grant, who lived on Long Island in Lawrence, honed his financial skills as a broker on Wall Street.

Grant wanted no part of free agency when it appeared on the horizon in a December 1975 ruling by arbitrator Peter Seitz that essentially nullified the reserve clause, which bound players to one team for life. The ruling was upheld by a federal court in March 1976, and shortly thereafter, Major League Baseball and the players association agreed to allow players with at least six years of service to become free agents.

Seaver’s salary seemed a pittance when Andy Messersmith signed a three-year deal for $1 million with the Braves and Wayne Garland got a 10-year, $2.3-million pact with the Indians.

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The Yankees were flourishing in the Bronx under free-spending owner George Steinbrenner, who in November 1976 signed Reggie Jackson to a five-year deal worth $3 million after signing Catfish Hunter to a five-year deal worth $3.35 million.

“Donald did not want to get involved with free agency like Steinbrenner was,’’ former Mets pitcher Craig Swan, 66, said from Fort Myers, Florida. “We were just getting retreads mostly.’’

GRANT TAKES HEAT

In February 1977, Seaver openly complained about Grant not spending money to improve the Mets, who would finish 64-98. Grant reportedly called Seaver “an ingrate.’’ De Roulet Bullock, who then worked in the Mets’ public relations department, said her family supported Grant’s position. “Everybody felt strongly, a contract is a contract is a contract,’’ she said.

Lorinda deRoulet, 84, declined to comment from her home in Manhasset. Grant died at 94 in 1998.

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The National League was taken aback by the trade. “Everybody held their nose about it,’’ said Louis Hoynes of Glen Cove, the league’s lawyer at the time, who was a friend of Lorinda deRoulet’s. “I think it was a miscalculation by Don Grant . . . He was on the wrong side of history.’’

Most in the media portrayed Grant as a curmudgeon. With withering attendance at Shea Stadium, the ballpark was called “Grant’s Tomb.’’ Former Newsday columnist Joe Gergen referred to Grant as “chairman of the bored.”

“He got off on the wrong foot with the sports press,’’ Michael Grant, 79, said of his father. “They pictured him as a member of the elite moneyed class, a stuffed shirt. He was anything but that. He was a guy that came to this country in the Depression and made his way. He never even graduated from high school. He’s a guy that really founded the Mets and fought for them when the National League [Dodgers and Giants] left New York and went to California.’’

Seaver wasn’t the only Met trying to get the team to buy into free agency. “I made comments along the lines that I wish there would be something done to improve the ballclub and it was taken as someone who is disgruntled and wanted to be traded,’’ pitcher Jon Matlack, 67, said from upstate Johnsburg. He was sent to the Texas Rangers after the ’77 season.

“It’s just the owners not knowing the economics of the game,” said former catcher John Stearns, 65, speaking from Aurora, Colorado. “They were old-school, where players shouldn’t make any money, and within a few years, salaries skyrocketed. Seaver should have never been traded.’’

A day before the trade, Daily News columnist Dick Young wrote that Seaver was envious of former teammate Nolan Ryan, who was making $300,000 with the Angels. “That galls Tom,’’ Young wrote, “because Nancy Seaver and Ruth Ryan are very friendly and Tom Seaver long has treated Nolan Ryan like a little brother.’’

That put Seaver over the edge. “That column was the straw that broke the back,’’ Seaver was quoted as saying in 2007. “Bringing your family into it with no truth whatsoever to what he wrote. I could not abide by that. I had to go.’’

Seaver also said he could no longer work under Grant.

Michael Grant, a financial adviser who lives in Riverside, Connecticut, said his father “thought he had done well by Seaver and that Seaver had rejected him. He would stand fast in the fact that Seaver traded himself. It had to taste bad that when Seaver got away, he was going to be smeared with the responsibility of it all. He felt bad about being labeled a villain. He went out as a villain, as somebody who was a terrible baseball executive. It was just sad that in the waning days while he was still chairman, he needed a bodyguard to walk around Shea Stadium.’’

M. Donald Grant was fired after the 1978 season by Charles Payson. The team was sold to Doubleday in 1980. Fred Wilpon had a fractional percentage but became the principal owner in 2002.

Michael Grant said that long after the trade, he and Seaver remained friendly neighbors in their Greenwich community but that the rift between the pitcher and Donald Grant “never got healed.’’

JOINING BIG RED MACHINE

Seaver joined a Reds team that had won the 1975 and 1976 World Series. “All of a sudden, we have Tom Seaver,’’ former Reds pitcher Jack Billingham, 74, said from New Smyrna Beach, Florida. “But to me, Tom Seaver is a Met.’’

Seaver spent six seasons with the Reds and pitched a no-hitter on June 16, 1978, but the Reds did not win another title during his tenure.

The players traded for Seaver quickly understood the magnitude of the deal from the New York perspective.

“When you pick up a newspaper and it says ‘Midnight Massacre,’ it kind of gives you an idea,’’ said Flynn, a banking executive in Louisville, Kentucky. “I can understand what was going through the minds of the fans. They were getting rid of The Franchise.’’

Flynn spent five years with the Mets and had a batting average of .234. He won a Gold Glove in 1980. Henderson showed an early flash, hitting 12 homers in 99 games in ’77 and finishing second in the Rookie of the Year voting to Andre Dawson, but he totaled only 25 homers in the next three seasons and was traded to the Cubs for slugger Dave Kingman. Norman played in 139 games over four seasons and ended his career with the Expos in 1982.

Flynn, 66, said it was impossible for the quartet of players to replace Seaver. “Y’all kept reminding us of it, so it was hard’’ not to think about it, he said. “I got cut from a college team and then signed from a tryout camp. And then all of a sudden, to be thrown into that situation. To be compared to what Tom had done would be silly on our part.’’

EPILOGUE

With the Mets now owned by Doubleday and Wilpon, Seaver, then 38, was reacquired from the Reds in 1982 for Charlie Puleo, Lloyd McClendon and Jason Felice. But in the winter of 1984, the Mets again lost Seaver. This time they left him unprotected in the free-agent compensation pool and he was selected by the White Sox. He won 31 games in two seasons for Chicago, including his 300th win, which came at Yankee Stadium.

Seaver pitched his last big-league games with the Red Sox in 1986 — the year the Mets beat that team for their second world championship. A knee injury sidelined Seaver in the Series.

In June 1987, Seaver tried a comeback with the Mets but officially retired after taking a drubbing by Triple-A Tidewater in an exhibition game.

After his playing days, Seaver was a color commentator on television for the Mets but had more popularity when he teamed with Phil Rizzuto on Yankees telecasts from 1989-93. The Mets retired his No. 41 in 1988 and he was elected to the Hall of Fame in 1992 with the highest percentage of votes (98.8) until Ken Griffey Jr. (99.3) in 2016.

Seaver’s relationship with the Mets nowadays is unclear. Last season, his wife chastised the team for not erecting a statue of her husband, saying in a published report that she was ”embarrassed’’ for them. Sources say the Mets would like to honor Seaver ahead of the 2019 reunion of the ’69 Mets, but no date has been announced. Nancy Seaver did not return calls.

“I think he just doesn’t like New York anymore,’’ Harrelson said. “He just doesn’t want to come back.’’

Perhaps he feels disenfranchised.