Originally posted by LesDMess The 1:1.2 was never released in a Pentax-M version while the 1:1.4 is available in the original K as well as M variation.
I believe that one difference between the K and the M lenses is that the smaller K's standardised on a 52mm filter thread while the smaller M's (and A's?) standardised on a 49mm thread. This is despite the fact that the previous smaller "SMC Takumar" lenses (the last screw-mount ones) had used 49mm filters*. So, unless you have special reason for a F1.2 lens for your LX, M or A-series camera, it would be a PITA as a standard lens with regard to filters. While I have a semi-permanent skylight filter for each lens, I prefer to buy only one red, soft focus or other special filter to share between them. The A-series F1.2 also had a 52mm filter thread so I guess the front element was just too large to squeeze within 49mm.
I am of course using the term "K lens" loosely. As seen in LesDMess's picture they were never called that (unlike the M's which were called "Pentax-M"). The lenses contemporary with the K-series cameras were called "SMC Pentax".
* I don't think there ever was a SMC Takumar F1.2 lens