At last! A question I can help with.
The answer is it depends
I know of two versions, both of which I've used.
One version is the type which had a flange on it for fitting and removing the adapter by hand. This version does not allow infinity focus as it adds between 1mm and 1.3mm of metal between the back of the lens and the mount.
The second version sits flush with the K mount and comes with a tool to remove it as there is no way of getting it out with your hands. This version allows infinity focus and works brilliantly. It effectively turns an M42 in a K mount lens. I;ve even gone to the extent of buying several and glueing them onto 2nd copies of my favourite lenses so I have in effect an M42 and K mount version.
The second version is the one I would recommend. Of the two, the flanged version is black and the other silver (though that might just be my examples). They are both very well made and should last, certainly more so than the cheap Chinese ones which you can get.
I hope this help,
Ryan.