Brian has it right. Another thing to keep in mind (not that it has a huge effect to this crowd, but I think it is still is worth knowing): The Zenit M39 SLR thread mount (example: the 1958 Brussels lens) and the M42 SLR mount had different flange to film measurements, so a 39mm lens will have limitations if used with a thread converter on an M42 SLR. If you just use a thread converter, the M39 mount lens won't ever focus to infinity, and the distance scale will always be off by a notable margin. But macro work will be just fine for you.
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