So different M39 mounts: a story about Russian M39

Now I am selling at Pinkoi and InspireUplift instead of eBay.
Here are the links:

https://en.pinkoi.com/store/russian-p...
https://www.inspireuplift.com/shop/Ru...

Everything as usual: economy shipping with tracking number, 3 business days as handling time, very careful packing, honest description, true photos. USA, Canada and Europe are also welcome.
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Usually when saying "M39 mount" we mean "Leica Thread-Mount".
But USSR produced several M39 mounts.
In this video I tell about Russian M39 mounts, their history, compatibility and how to distinguish them.
Language: English
Note: some technical details are simplified.
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Video fragments used:
"Педагогическая поэма", Киевская киностудия, 1955
"Республика ШКИД", Ленфильм, 1966
"Бомбардировщики. Холодная война", Крылья России, 2013
"Улица полна неожиданностей", Ленфильм, 1957
"Все наоборот", Центральная киностудия им. Горького, 1981
"Берегите женщин", Одесская киностудия, 1981
"Солдат Иван Бровкин", Центральная киностудия им. Горького, 1955
Music used: VedernikOFF - Celebration