Both rings have 4mm thick flanges for convenient usage.
So, you have three options of this adapter use:
Mount a microscope objective with RMS/DIN (4/5"-36tpi thread) on LTM camera (Leica, FED, Zorki, even some enlargers);
Mount a microscope objective on cameras with unversal M42x1 mount thread (Zenit, Praktica and many other cameras includind modern DSLRs since there are plenty of M42-to-something adapters);
Mount a M39x1 (LTM) lens onto M42x1 camera or M42-to-something adapter. Flange focal distance isn't adjusted, so your M39 lens will not be able to reach infinity focus and will work in macro mode only (in about 20-30 cm range). But primary use of these adapters is microscope objectives, so you are enthusiast of close-ups anyway.