I found this out when I got my Pentax 110 18 and 24mm lenses. No pics yet, I am letting the glue dry completely. The 24mm lens has 25,5mm filter threads and came with an original plastic sky filter. I had another filter lying around.
So one filter screws into the C-mount adapter, another needs to be shaved down a bit to achieve infinity focus. The shaved down one is glued to the lens and screwed into the other filter ring. I can even put an aperture disc in between!