never heard of a 120mm, sounds like a bit of an oddity.
Pentex made 42mm lens from 35mm to 150, for the most part, in 15mm increments, although from some reason they jumped to 105 rather than 100: 35, 50, 85, 105, 120 135, and 150. The 120 is somewhat rare, the 150 is easy to find. I find the 150 to be a better fit than the 180, I have a sologair 180 in 42mm as well, fast but just close to a 200. I dont think the 120 and 150 were carried over in the K lineup.