Originally posted by Blue Basically, Chinon and Ricoh were using the K-mount with some sort of licensing agreements. Ricoh actually had a slight variation but those lenses would work on Pentax cameras. Sears had some decent lens in what was called a pkr mount. Sometimes those lenses can cause problems on the new digital Pentax bodies but sometimes not. I have 3 of those lenses. I have 2 copies of the model 202 135mm f2.8 lens, one got stuck when I tried to use it on my K200d and the other didn't. I strictly use them on the film bodies because of that.
Some people have removed the Ricoh pin but I didn't want to do that. I have the Sears 80-20mm and the Ricoh pin causes problems on the k200d as well.
Anyway, when Chinon went autofocus, the lens compatablilty shifted. Those Chinon AF lenses won't work on Pentax bodies. Vivitar also had some cameras made with k-mount bodies and Sigma did as well. Sigma also has a mount of their own. Ricoh and Chinon are usually the best bet for film bodies as long as you stick with the manual focus bodies. I think you can use mf pentax lenses on the AF Chinon bodies but am not sure.
Pentax left the original K mount open for anyone to use without a license. That is why there were some other companies like Chinon, Sears, Cosine, Ricoh... who used it for their cameras and lenses. It makes it more fun being a Pentaxian since it leave us more old lenses to try out.
When the KA mount came in 1983 and later the KAF, they were patented and not open, which forced Ricoh and Chinon to try their own solutions...rather unsuccesfully.
As for Zeiss involvement in the original K mount I know no really reliable source (Wikipedia for sure isn't without a reference). It is possible since they did colaborate on the super multi coating and on some lenses (most know the 15mm/3.5), that was just prior to (SMC) and just during the shift from m42 to K (the 15mm/3.5). But it could also be a rumor. Even today there is a tendency from some people to try to give the credit for all current lenses to someone else, like Tokina (while actually Pentax designed all the lenses that you can get from both Pentax and Tokina except the 12-24mm which is Tokina design).
Remember that Pentax went bayonett rather late and hanged on to the m42 mount (improving it with automatic apperture transfer) in many years when the competition already had bayonett mounts. The advantage that we have is that they kept the same registration distance for the K mount as the m42 lenses have (and the m39 lenses built for the Asahiflexes). That's why it takes such a simple adapter to mount m42 in K cameras.