obviously it's been banging around in there loose for a long time & the trans was working OK (for the most part I'd think) for quite awhile for it to have been up & running long enough to do that amount of long term damage. As oldmanmopar said if a caged ball bearing cage lost a ball it've shredded. To me it sounds like either a ball bearing got loose that wouldn't affect the trans operation & I'd think all of em would need to be accounted for for proper operation or an extra ball from another trans at a tranny shop got dropped inside during the overhaul but that dont seem likely . That mystery may not be solveable. Myself I'd open it up including the valve body & verify that every ball is there that is supposed to be there & if so button it up & call it good.