M42x1 would be the same regardless of whether it is photographic
equipment or heavy machinery since it describes a thread with a 42mm
diameter and a 1mm pitch. There is no guarantee that something (other
than camera equipment) described just as M42 wouldn't be M42 x 0.75 or
M42 x 1.25, but I would bet that any M42 lens would be M42x1.
Accrording to wikipedia Leica thread mount is M39 x 26tpi (whitworth)
which works out at M39x.977. Apparently early Zorki and early Canon RF
cameras used M39x1.
It gets really murky in the world of large format lenses and shutters
where some of the metric sizes are very close to imperial sizes and
you need to measure very carefully to work out exactly what you have.
Prior to WWII there weren't any real standards and lots of oddball
sizes exist.
Paul
On a related note, is what we call m42 the same as what machinists
call M42?
http://www.besly.com/catl/catl4124ps.htm#M42
Yes, I think so. Leica screw mount is M39.
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