Got around to trying the grip parts swap between a Pietta cap and ball '51 Navy and a Uberti SAA with birds head grips/frames. Actually I was just trying to put the Uberti grip frame on the Pietta and made no attempt to try the other way. The screws stayed with the parent frames as suggested by Capt M earlier. The good, all holes did line up on both grip frame sections, kinda!
The bad, the first two screws I tried to put in were the two behind the trigger guard and they did not seem to want to thread in, even though these were the screws original to the frame. I quickly figured the problem, the angle at which the two parts hole alignments were driled are not on the same plain. By lifting the trigger frame slightly I could get those two screws in but before they tightened down the front of the trigger guard was angled well up. It was easy to start a few threads on all three of the trigger guard screws but I am sure drawing them down would cause considerable flexing and probable bending.
The worst, my best option left was to leave the Navy trigger guard frame on and see if the birds head rear grip frame would attach. First I had to leave the wood off as its channel was slightly to narrow. The two top screws liined up great but though the base screw holes can be hovered directly over each other the frame halves are separated to far to be called a fit and the mating surfaces were no where near a match either.
Back to the good, a slight bit of work to get the two holes, mentioned first, bored in alignment and I think the entire assembly would work pretty well but the two grip halves need to stay matched, P to P and U to U. One last thought, when I pulled the trigger guard off the Pietta, I noticed that pietta has a very slightly raised shelf that the guard rests on and the Uberti SAA clone does not? I will have to look at my other Pietta clones to see if this is common to them all but I do not have Uberti Navys to check.