As far as I know, Pentax never made an M-series 24mm, which is a great shame as 24mm is a focal length that I use a lot. My disappointment stems from the fact that because I use M series lenses plus the standard (non-M) 24mm the difference in filter size can be a hassle, meaning I have to take two sets of filters (49mm & 52mm) and can't just swap lens caps when changing lenses.
I wonder why Pentax didn't make a 24mm M? Perhaps it was an issue with re-packaging the existing optics. They did, though, make a 20mm M-series lens with 49mm filter size, which is very useful on occasions.
Steve