Hmm. I know some were made in K mount, I've never seen any in an F-mount though, other than surgical jobs like yours, so good job :-D I had an M42 Helios 44 I used to like shooting with no infinity focus on various F bodies. I eventually bought a spotmatic specifically to shoot it with. Then picked up some takumar lenses, then a couple more spotmatics and then ... well you get the picture. I could have saved myself an entire extra camera system had I done this modification originally to my 44 :-)