The lens is mounted inside a tube. The inside diameter of the tube is
60mm and threaded. Those threads accept a 60mm retainer ring (that
holds the lens in place).
The filter I have is a Leica 60mm dia. polarizing filter which is able
to screw into the remaining threads at the end of the tube. I only
mention this filter so that one would be apprised that the thread specs
are the same for both the ring and the filter, and I assume are
standard.
mg