Billy Conn turned pro having never contested an amateur bout. That probably at least partially explains his sketchy early career form. There's no doubt, that after time learning on the job, that he went on to have an ATG career.
Chris Billam-Smith regularly would go out in the semi finals, quarter finals or in the final of the national championships in the amateurs, never winning. And wasn't good enough to get on the GB squad. As a pro he's won British, Commonwealth, European and a WBO world title Far surpassing everyone who beat him in the amateurs.
The greatest fighter of his generation, Julio Cesar Chavez, had but 13 or 14 or 15 amateur fights depending on whom you believe.
If I’m not mistaken… Terrance Crawford was somewhat of an unknown as an Amateur…and it was Mikey Garcia who told their promoter about him …(correct me if I’m wrong)
Jeff Chandler thought got robbed in his second amateur fight, he felt so disgusted that he did away with the ammys entirely and turned pro. Even got his get back against the same guy he lost too!
I don't believe Henry Armstrong had an amateur career. He saw how much a world champ made in the paper and quit his job the same day.