Reliever Jack Aker of the Athletics, who is one-eighth Potawatomi, is supplying a new twist to the old cowboys-and-Indians plot. Take last week. Jim (Catfish) Hunter of the Athletics was pitching the finest game of his career against the Orioles. Hunter had a 3-1 lead in the bottom of the ninth, when he suddenly got into trouble by giving up a single and a walk. There were two out, but Hunter's first pitch to Woodie Held was sky-high and there was hardly a soul in Baltimore who was not confident that the A's, true to their traditional second-division form, would blow the game. Then out of the bullpen came Aker. He is a former junior college fullback and a former minor league outfielder who hit so poorly that, although he had been only a mediocre high school pitcher, he was advised to go back to pitching. Despite his unimpressive background, hitters now have an abiding respect for Aker, who last year tied a major league record with 26 saves. In Baltimore, his warmup pitches out of the way, Aker quickly disposed of Held on a routine grounder and earned his second save of the season. Three days later he drove in the lead run in the eighth inning, then once again stopped the Orioles cold as he won his second game of the year. Jack is called The Chief, The Savage and Half breed by teammates, and less complimentary names by opponents. When The Chief comes to the rescue (forget the cavalry) he leaves American League batters feeling like so many General Custers.