For the C&R I believe the stock must also be original production. Some have reproductions around that are not easily discernible from originals and I believe burden of proof is on ATFE, but i wouldn't play that game if I were you.
Here is an angle on this: You have a BP stocked pistol. YOu take off the stock and add a conversion cylinder without the stock attached. Have you broken the law if the stock is in your possession? Can you then re-install the original BP cylinder and stock?
This is one of those interestingly stupid laws.