Just found a similar one on gunstar, that one is a 1857 Tower so only a year difference. That too is a 0.60 caliber.
I know you said you think it was re bored but I would of thought there would be traces of the old inspection mark for the gauge if it was re proofed. So could it be that some were indeed made to 0.60 caliber from the factory? Maybe a certain unit requested this specific caliber? Either way I dont plan on shooting it anytime soon and it very much a S58 wall piece. Might be different in the future but yes, I rather like not being blown up so i would get it checked out before hand