Jerry Koosman won the clinching game of the Mets’ first championship, no small feat, going the distance to defeat the Orioles, 5-3, in Game 5 of the 1969 World Series.

Yet as someone not yet around for that seminal moment, I feel like I hear more about a different game that Koosman pitched that season — which speaks to both the magic of that Mets campaign and Koosman’s vast contributions to it.

For when I had the honor last week of chatting with Hall of Famer Fergie Jenkins about Koosman, he brought up that same game.

“He pitched against us late that season and got a big win, against Bill Hands,” Jenkins said. “When you’ve got a pitcher that knows what he’s doing out there, the games aren’t going to be long.”

This game, a 3-2 Mets victory over Jenkins’ Cubs on Sept. 8, 1969 at Shea Stadium, lasted two hours and nine minutes.

Free advice: Bet the over this coming Saturday night, when the Mets play host to the Nationals at Citi Field. More free advice, especially to longtime Mets fans: Be seated by 6:45 so you can watch the Mets retire Koosman’s number 36 in a ceremony that will feature Koosman’s 1969 Mets teammates Wayne Garrett, Cleon Jones, Ed Kranepool, Art Shamsky and Ron Swoboda.

“It’s a very humbling feeling when they called me and told me they were going to do it,” Koosman said to The Post. “I didn’t realize I accomplished enough to get it done. But evidently, somebody recognized it. It’s a nice career accomplishment. I’m very pleased and excited about it.”

The 1969 team was of course the Miracle Mets, in its eighth year of existence and having posted a franchise record of 73 wins in the season prior. They stood as far back as 10 games out of the National League East penthouse, which was occupied by the Cubs, on Aug. 13. Hence the shock when they won 18 of their next 23 matchups to pull within 2 ⅓ games of the Cubs, just one game behind in the loss column as Leo Durocher’s squad arrived in Queens.

Hands, a Rutherford, NJ native almost done with a fantastic season — he tallied a 162 ERA+ in 300 innings — began his night with a high and tight fastball to Mets leadoff hitter Tommie Agee, as per reports. Koosman responded by drilling Ron Santo, the Cubs’ first hitter in the second inning, and then striking out the next three batters: Ernie Banks, Jim Hickman and Randy Hundley.

“You’ve got to do it,” Koosman said, acknowledging the intent of his pitch to Santo (We’ve presumably reached the statute of limitations for Koosman, now 78, to be disciplined by the commissioner’s office). And the Mets proceeded to win 3-2, to pull even in the loss column, and the next night, Jenkins fell short to his fellow Hall of Famer Tom Seaver, 7-1, in what is best recalled as the “Black Cat Game.”

Jenkins said he always suspected that Mets closer Tug McGraw, a noted jokester who passed away in 2004, was the one who let that cat loose. We’ll probably never know.

What we do know is that the Cubs, who at 84-58 still led the Mets (82-57) by a half-game after leaving Shea Stadium, never recovered from that one-two punch of Koosman and Seaver (and the cat). Chicago lost to the Phillies the next night, and the Mets swept the Expos in a doubleheader to go up by a game. The Mets wound up winning the division by eight games.

“It was getting exciting because we were winning and we overtook the Cubs in that series, right?” Koosman asked. Close enough. “We felt very competitive, like we were going to, at that point, go on and win.”

The rest is history, and Jenkins, his number 31 retired by the Cubs as a tribute to both him and his fellow Cooperstown honoree Greg Maddux, fully supports the Mets’ decision to show the same respect to Koosman.

“I thought he got overshadowed a bit by Seaver,” Jenkins said. “He had good control. A good curveball. He was one of the best left-handed pitchers of that time.”

He’s one of the best Mets ever, too, and now that’ll be more apparent to all who come to Citi Field.