Good morning, Marc;
I have several mirror lenses. For "f stop" adjustment, they all use neutral density filters. Most of them screw into a provided filter mount in the lens mounting flange where they are accessible. One of them has a small turret assembly that puts an ND 2X or an ND 4X filter into the light path just in front of the lens mount so that it has "f stops" of f 8.0, f 11, and f 16. Because the glass of the neutral density filter will also affect the light rays, there is also a "normal filter" that is made of clear glass that is screwed into the place for a filter to provide a common element of refraction to insure that the light rays all go to the same place on the film plane.