For a few pitchers, their 300th victory pulled them out of the Hall of Very Good and punched a ticket to Cooperstown.

I’m thinking primarily of Don Sutton and Phil Niekro, who probably would have been shy of immortality had their career-wins total begun with “2” rather than “3.” Whereas Tommy John (288), Bert Blyleven (287) and Jim Kaat (283) are still kept out of Cooperstown largely because they didn’t earn those scant few extra victories.

For Randy Johnson, who became the 24th member of the 300-win club Thursday by defeating the Washington Nationals, mounting a Hall of Fame case long ago ceased to be an issue.

Reaching 300 merely put a shiny bow on Johnson’s already impeccable Hall of Fame credentials — including, but not limited to, five Cy Young awards, two no-hitters (including one of 17 perfect games in history), co-MVP of the 2001 World Series and 4,791 strikeouts (more than any pitcher in history besides Nolan Ryan, Johnson’s mentor and the man who turned his career around with a hands-on tutorial in 1992).

In fact, a strong case can be made that Johnson is the greatest left-hander in history — and you have no idea how much that pains this lifetime worshipper of Sandy Koufax.

Koufax was, is, and always will be my baseball idol, and I remain convinced that in his prime, he was the greatest pitcher, left- or right-handed, who ever lived.

But Koufax’s prime was sadly cut short by the chronic arthritis that caused him to retire, at age 30, after the 1966 season — a year in which all Koufax did was go 27-9 with a 1.73 earned-run average and 317 strikeouts in 323 innings.

Johnson, remarkably, has won more games in his 40s (72-50 in 156 starts) than his 20s (64-56 in 158 starts) — a testament both to his durability and how erratic were his fledgling days in the majors, when he had to learn to hone the mechanical challenges uniquely brought about by his gangly 6-foot-10 frame.

He did so, of course, to an overpowering and intimidating extent, and now belongs in the pantheon of southpaws that includes Koufax, Steve Carlton, Warren Spahn, Lefty Grove, Whitey Ford, Eddie Plank, Carl Hubbell and Rube Waddell — not to mention the only other active 300-game winner, Tom Glavine, who was released Wednesday by the Braves.

Assuming Johnson calls it a career after this season — and having fulfilled this final milestone at age 45, with his skills in undeniable decline, it’s hard to imagine what would make the Big Unit keep going — I’m already gearing up for what could be the induction ceremony to end all induction ceremonies in July 2015.

Let’s say Johnson hangs it up after this year. And let’s say, for the sake of argument, that Ken Griffey Jr. does, too. I have no inside knowledge here, but it’s reasonable to think that Junior, having achieved the closure of coming back where he started, might decide in November that it is time to walk away, at age 40.

Barring any new revelations, these two are first-ballot no-brainers. And that means that Junior and the Unit, who were rookies together in Seattle in 1989 and grew up together with the franchise, would go into the Hall together. Not since Mickey Mantle and Ford in 1974 have two players so closely linked gone into the Hall of Fame in tandem.

Johnson and Griffey were teammates in 1993, when the Mariners finally broke through with the first winning season in franchise history, and two years later, when the franchise won the hearts and minds of the community in the “Refuse to Lose” miracle of 1995. Johnson’s performance out of the bullpen in Game 5 of the Division Series, and Griffey’s dash around the bases, have both become the stuff of legend.

Johnson’s ugly departure in 1998 muddied the waters, but now in his final days, he has shown a newfound sentimentality for the early days in Seattle, even seeming to prefer that he go into the Hall with a Mariners cap.

No matter which cap he wears in Cooperstown, Johnson’s Seattle legacy is undeniable. And his baseball legacy now is complete as well.