Young Jack Dempsey & Meyer Grace are the same guy.
However, some bouts on the Young Jack Dempsey (alias jack Grace) are for a heavyweight and are not the same fighter. I don't know where the name jack Grace came from -- I suspect he may be an altogehter different heavyweight fighter.
So, anyone want to move the welter bouts for Young jack Dempsey/jack Grace to Meyer Grace, leaving the heavyweight bouts, unless there is proof positive those bouts were Meyer Garace, a welter fighting heavies.