Bob, I've never tried to swap original tangs into a C. Sharps, or the other way around,  but I did once try to swap tangs between two of my CSA 1885's ( a straight grip and a pistol grip ) and it was a complete no-go.  The dimensions were different enough that it was impossible to slide them into the action.  It wasn't even close, so I expect that there is a fair bit of hand fitting done to these.

Chris.