This is interesting, here's a Russian classic-car restorer that restored a GAZ-A engine that apparently they found at a WWII battlefield:
http://simonovmotors.ru/works/3/
Here's the shot of the right side of the engine block. No insignia near the oil return boss. Also note whatever's going on at the lower edge of the valve chamber.
worksimg59.jpg
Not to say this
isn't a very early casting, but so far the evidence tying it to GAZ/NAZ seems weak to me.