One of the rarest Topcon accessory is the UV mount adapter. If you look at a UV mount lens, it has no aperture control ring. Works on a UV mount camera as the camera has an aperture ring on the mount. the other Topcon cameras used the R or RE mount (based on exakta) so they made an adapter. Not that many were sold, or maybe they got discarded as people though it was just random lens parts… at any rate they rarely show up for sale anywhere.