it's just that his is a Uberti R&D cylinder and IT won't fit in a pietta
I just pulled out both a 1858 Pietta and a Uberti Rem and put re Uberti cylinder right on it in about 2 blinks...cylinder pins are the same size.
Besure you rotate the cly. clockwise as viewed from the grip end to move the hand that protrudes from thr frame forward... if it's a Uberti in a Pietta there s/b no cly. length interference at all the Uberti is aprox. .010" shorter than a Pietta...cyl. face chamber end may need some lappin' or is R&D send them back and they will fit...specify do not adjust the frame, just adapt cylinder.