If you have the gun timed to that cylinder the C&B cylinder will then be out of time.
Honestly, I don't care about the C&B cylinder. I will rarely shoot C&B if I have the conversion cylinder. I just want the conversion cylinder to be in perfect time so I don't have to "worry" about it when I shoot it.
For the record, I have a Uberti and a Pietta 1858. The R&D is a Uberti model (even stamped with a U on it). I just want it to be in time while in the Uberti. It doesn't fit (basepin hole on the cylinder is too small) the Pietta.