I also found what are labled nel-spot barrel extenders at my local army navy store. they are a black aluminum tube w/ 2 allen skrews on the side. IF anyone could enlighten me on these I think that they are made for the nelspot 007 because vintage rex said thet the 007, not the nelspot, had a removable barrel sleeve the allowed a barrel extender to be used . Also thay have 14 and 12 inch brass barrels that look funny on the ends they might be for a nelsopt conversion. The barrel look like this sorry about the drawing.
PM me if you want pics.