[GUB]

Oh and if enthdegree revisits I did it with a short K mount ext tube with the male end removed. I bonded the female end of a helicoid into the recess left by the removed kmount. Minimum length is just over 20mm but is still great for m42 bellows use.

[kypfer]

Further to the bellows idea ... B.P.M. (in the UK) used to make a "Universal Bellows Unit" with, believe it or not, interchangeable mounts! Perfectly practical to have a K-mount at one end and an M42 mount at the other, or virtually any other combination. The unit was even supplied with a basic slide-holder to clip onto the bellows rail