IIRC, the application form doesn't address the type of stock, but does ask the overall length. So if you changed from a full stock to a pistol grip type, doesn't that change the OAL, which changes the information in the NFRTR? Is that OK to do without notifying BATFE? Or if you want to switch back and forth, what are the legal ramifications?
At one time, I would have said that BATFE wouldn't care, but today it could be seen as invalidating the registration and/or committing a felony by lying on the Form.