Did Tim or Fireball ever check the date code on the lever boss for that gun? That should indicate the assembly finish year.
My presumption with that rifle was that it had probably been finished late in 1960 and they'd bored an already stamped Chicopee Falls barrel blank to 250-3000 with 1-10" twist. But while possible, that seems less likely now. Hey, I've got a 99DL in 284 with a Chicopee Falls barrel, so don't tell me it can't happen.
David from that thread made a good guess..:
Maybe Savage started the one in 10 twist in 1959 rather than 1960
