FYI, for what it's worth, while the Minolta C.E. lenses (80mm, 50mm & 30mm) do not have front filter threads, they do have the same front diameter. I discovered long ago that most 55mm filters fit perfectly over the front of the lens. Just unscrew the glass retainer. Small dabs of silicone sealant will keep it in place -- and the Minolta lens make great close-up lenses. The 55mm filter thread allows you to use whatever filters you want -- and to reverse mount the lens at higher magnifications.
One word of caution. The C.E. lenses have two light pipes which need to be covered up to avoid stray light during the exposure. This is easy to do with small pieces of black tape inside the lens mount -- or, if you can find one, Minolta made an adapter that covers them up.