Louis Clark Brock got two singles in a baseball game in St. Louis Monday night. There is nothing unusual about that -- he has been doing it for years. But these were hits No. 2,999 and 3,000 of his major league career. That, as every sports fan knows, puts Mr. Brock in one of baseball's special categories. As important as those 3,000 hits are to the keepers of records, however, they are not what makes Mr. Brock special. There are other things.

One of them is that Lou Brock and a tiny handful of other players restored to baseball in the 1960s something it seemed to have lost. Unlike the strong men whose bats propel balls over shortened fences, he was a player whose success depended upon speed, skill and finesse. He was a terror on the base paths and reminded fans there is as much joy (and anguish) in stealing a base as there is in hitting a home run.

The other thing about Mr. Brock is this: When he got those two hits the other night, he was almost two months beyond his 40th birthday. Some of us find that one of baseball's more gratifying statistics.Folklore has it that men of his age are, as they say, over the hill. But this year, Mr. Brock at age 40 has been tearing up the league, hitting more frequently than ever before and outplaying men 10 and 15 years his junior.

It is the last year he wil do so. He announced last winter, and has stuck with it ever since, that this summer will be his last on the playing fields. That announcement, if the reports from St. Louis are correct, brought grimaces from those who pay his salary. He has had a disastrous season in 1978 and they wanted him to quit. No man, the pundits said, could come back at 40 from so bad a performance and play well in a sport demanding such good eyes and quick reflexes.

But Lou Brock said he didn't want to go out as a flop. One more year, even at 40, and perhaps, just perhaps, he could go out on top. He got the year, he is having the season he longed for, and he is going out where he wanted to be. That's class and that, more than the 3,000-plus hits and the 900-plus stolen bases, is what makes Lou Brock special.