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''He has a great arm,'' announced baseball great Stan Musial after President Reagan threw the first ball for a short game on the White House's South Lawn the other day. Mr. Reagan had allowed that the occasion - proclaiming National Amateur Baseball Month - was more fun than being President. After all, baseball is the national pastime, ''next to political campaigning.'' It would be nice if ''they'd set aside a month for my defense budget.''

In other words, the President was in good form, giving equal time to both benches as little leaguers played with stars on the field - and in their eyes.