Thanks to everyone for the replies, it sounds like no one has tried it yet. I'll be the first, I guess, I'll post how it works after I find myself a J12 for my Zorki.
The Russian finders I've seen on Ebay go for $40-50usd, but I got the finder for my Yashica for $5 usd. If the Yashica works, I'll save myself buying the FSU one.

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Harold*
- at one with many rangefinders and a fine Nikon F2A