I’m sticking with my educated guess. I think it’s a repurposed 30-06 brass. As for what, I can’t say. I reckon there might be a rifle out there somewhere that would shoot it, lol, but I don’t think that case started life in its current state.
Definitely agree with you, it's a reformed 30-06 case. But formed into what? I've reformed '06 into 7.7 Jap, 7.65 Mauser, and even 8mm Mauser. Lots of 270, 308, 30-06, and anything else with that size case head have been formed into 7.92x33 for a friend's MP-44.