Having spent the last three months isolated in the unreal world of the Iran-contra hearings, a study in unaccountable free-lance political entrepreneurship populated by incredible characters, it is refreshing to reenter the world of accountable American enterprise this summer and to find that more familiar American hero types still abound.

Two in particular come to mind.

The first is an aging athlete whose career had been distinguished more by longevity than great achievement, until one recent glorious moment, that is.

He's Joe Niekro, a Minnesota Twins pitcher now toiling in his 21st big-league season. Niekro achieved a form of immortality the other day for the courageous, candid way in which he responded to great adversity.

Said Joe, after being caught apparently scuffing baseballs with an emery board and piece of sandpaper, making them do what the home-plate umpire called "odd things:"

"Being a knuckleball pitcher, I sometimes have to file my nails between innings, so I carry an emery board with me to the mound."

That charming explanation is so American, so guileless, so endearing that it qualifies him for nomination as Man of the Season. Not since the tear-stained boy pleaded, "Say it ain't so, Joe," to Shoeless Joe Jackson after the great hitter and his Chicago "Black Sox" were implicated in throwing the 1919 World Series has so innocent a cry been heard throughout the land.

In a second category, Best Summer Sourpuss, an overwhelming favorite is George Steinbrenner. He owns the New York Yankees, a club that used to have class and now has, well, Steinbrenner.

Said George, who already has alienated his players, including selfless Don Mattingly, the best all-around first baseman the Yankees have had since the incomparable Lou Gehrig, it's not just the players who are bums, it's the manager, too -- or words to that effect.

Just 16 hours after the Yankees slipped out of first place, gracious George issued a blast at manager Lou Piniella. Seems Piniella had the temerity not to be there when the owner called from out of town, prompting the great sorehead to remark, "I don't know of too many guys -- even sportswriters -- who when their boss told them to be available at a certain time, wouldn't be there." Then, demonstrating admirable consistency of character, Steinbrenner responded to comments by several of his players that he should let the manager manage the team and them play the game by saying:

"That's okay with me. We'll just try it that way, and we'll see how well they do . . . . They can put up or shut up. Maybe it's about time for it."

Coming after the depressing revelations of the Iran-contra hearings, this is reassuring. It's proof conclusive that the country has its values straight and raises the intriguing thought: As we approach the next presidential selection period, how about an Ollie North-Joe Niekro ticket? With George Steinbrenner as campaign manager, of course.

Unbeatable.

In a column two weeks ago, I described the dedication of a small Korean war "meditation bench" at Arlington National Cemetery and noted that it was the first such memorial there to veterans of a conflict that cost 54,246 American lives in just three years, close to the number of 57,685 battle deaths sustained by U.S. forces in 9 1/2 years of combat in Vietnam.

Several people, including members of Congress, have written to say that plans are under way for a national Korean War Veterans Memorial, probably in the Mall area, to be maintained by the Interior Department. A year ago, Congress authorized the American Battle Monuments Commission to erect the memorial here and provided $1 million in initial federal funding. Private fund-raising has begun for the estimated $4 million also needed, and President Reagan recently appointed 12 people to serve on an advisory board to guide design and development of the monument.

I've also learned, thanks to mail from around the country, of the existence of another "forgotten war" memorial in Memorial Park in Omaha, Neb.. Dedicated 11 years ago, it honors Korea and Vietnam veterans. In addition, there are plans for a memorial in Los Angeles to pay tribute to those who participated in the Chosin Reservoir campaign in Korea. These developments suggest, I'm happy to report, that the Korean conflict is not nearly as forgotten or ignored as I had thought.