FED-1 with a mount-like cut in the back, covered

I will spend a long week-end somewhere in Russia in a couple weeks from now, and as usual in such case, I am looking around at second hand online places and on Avito, for lenses and cameras.

A guy in SPb is selling a FED-1 (d) in Princelle categories, (III) in Suglob-Kochergin-Shaternik ones, ie. a NKVD-SSSR Kombinat of the years 1939-1941:



but this is the back: !???





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Current working cameras:
RF: Fed-2, Kiev-2. Zorki-4, Fed-4
MF: Salyut, Zenit-80. Salyut-S, Kiev-6S, Kiev-60, Moskva-5
SLR: Zenit-V, Zenit-3, Zenit-3M, Zenit-122, Start

Curious.
Of course, it would be interesting to know if the plaque is blind (just to fix the camera on a stand by its back) or is an opening towards the inside, for some reason.

No idea?

Jacques;

Just a photo of my Reid I, with an opening on the back, to be used on a periscope (the red markings are from the Royal Navy). But it is only an example: the Zorki 3 and 4, used on periscopes too, don't have the same system.



Amitiйs. Jacques.