Originally posted by Jüri One of the reasons that I use Pentax is defenitely the backwards complatibility, which no other DSLR brand has. However, the fact is that with Pentax I have to put the whole adapter inside the mount, while Y/C system allows to add the adapter easily or even put it on a lens, while retaining infinity focus. I thought there might be a particular reason the K-mount system was not designed that well considering the M42 compatibility.
Have you actually tried putting the adapter on the lens first and then mounting it on the camera? Removing the adapter will require removing the lens first, but that's another story.
The big deal with adapting M42 to K mount is that they have the same registration distance, so the M42 adapter needs to fit inside the K mount. This in turn means that it cannot lock to the body using the K-mount lock pin, so it needs that small tab to lock inside the mount and that in turn means that you cannot remove the adapter without removing the lens first to gain access to the tab.
I'm not familiar with the Yashica mount, but their registration distance and lens locking mechanism are probably different. Why didn't Pentax change the registration distance? Probably to save cost on changing lens designs. Why didn't they come up with a different locking mechanism that could be used by the adapter as well? I have no idea. They could indeed have done a better job in this area, but maybe they wanted to push people to buy new lenses, so they didn't waste time to make the adapter story better.