After a long pause I just bought a new (to me) lens I've always wanted to try. It's the famous Kiron 100/105mm macro. I found the Vivitar 100mm f/2.8 version in PKA mount, which is exactly the same lens as the Vivitar Series 1 2.5/105mm, Lester E Dine, Kiron... just with a slightly different build.
I found online suggestions that the former non-series 1 version could have been better, even a photo magazine comparative that tested the two lenses at the same time. I wouldn't be so sure about that, I tend to believe that it could have been due to sample variation, not a real optical difference.
Had little time to test it properly, but I already found that the sharpness is very good even wide open. With the diaphragm ring on A position it's a pleasure to use on my K-1.
I promised myself that this is the last 100mm macro I buy. This is the fifth, I already had way more than enough, but I have always wanted this one, so I made an exception :-)