The screws which attached the stock to the grip were socket head, and had the hex drilled out, so the stock was not readily removable.
I suppose the SFSS decided that the frame was a handgun frame, therefore the thing is restricted.
The OP asked about value. Well, if it is a handgun and isn't registered, it cannot be registered. If it is registered as a handgun it is not transferrable, therefore not saleable and has no value.