A reason Ricky Bell, the past couple of seasons with Tampa Bay and San Diego, wasn't the running back he used to be: an ailment just diagnosed as polymyositis. Bell, who carried the ball twice in 1982 and finished on Chargers' injured reserve, has begun taking cortisone for the illness, in which his muscles became inflamed while the joints swelled so severely he had trouble putting on his shoes . . . Florida convict Mercury Morris, the ex-Dolphin, flew to Pittsburgh yesterday under guard with his wife and baby to visit his mother, who has terminal cancer; he would spend the night in Allegheny County jail between hospital trips . . .

In Morganton, N.C., George Moore was kidding Paul Rutherford when, the sheriff's office alleges, Rutherford pulled a knife and chased Moore, who landed in hospital with multiple stab wounds. Moore had been razzing Rutherford, an avid Dallas fan, about the Redskins' victory over the Cowboys . . . The Redskin rooters' rally set for 6 p.m. Saturday in L.A.'s Westin Bonaventure Hotel is shaping up big, with WMAL cosponsoring; Redskinettes, top civic officials, Over The Hill Gang alumni expected--and efforts afoot to squeeze in Myrna Hog, a 1,400-pound Hollywood sow.