The American League's designated-hitter system is normally straightforward enough: the DH bats and the pitcher doesn't. Last week a wrinkle in the rule produced a bit of baseball history. It began when Reliever Cy Acosta of the White Sox came in against the Angels in the top of the seventh of a 2-2 game. In the bottom of that inning the Sox scored six times and Manager Chuck Tanner made a defensive move, shifting his DH, Tony Muser, to first base in place of Dick Allen. Ah, but when a DH takes someone's place on defense, only eight hitters are left in the lineup. That means the pitcher must hit for the player who has been removed. Thus Acosta batted for Allen in the eighth inning and became the first AL pitcher to appear at the plate in 1973. The Sox bench rose to cheer him. Owning neither bat nor helmet, Acosta borrowed them. He then struck out, illuminating, however unheroically, the reason they invented the DH.