About a year ago I was contacted by a gentleman who owned a very similar rifle.
He had done a chamber cast and dimensions matched very well.

Yes, this cartridge predates all the short cartridges for assault rifles.#
But the bullet clearly was intended for hunting.
Now I found this rather new Russian cartridge
https://modernfirearms.net/en/cartridge/366-tkm-2/
Not clear to me if it uses .366 or .375 bullets.
But otherwise a full circle back to where it started

Here is a link to an earlier thread in the Nitro Express forum. So we discussed this gun serial no. 1 already in 2019, and it was on GunsInternational back then.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=326646&an=0&page=4#Post326646

In this discussion a link to a third rifle came up
https://vintagesemiautorifle.proboards.c...nauer-automatic

Interesting situation, I know of 3 rifles but only one cartridge.

fuhrmann