With the same OAL but a larger dia shoulder, I'd suspect a rechamber to 8x57 Ackley Improved or some other less known but similar wildcat of the day.
The A/I shoulder is supposed to be .456 IIRC. But being a wildcat cartridge, reamer dimensions varied. Home-made reamers and one-of builds can give you some interesting combinations.
The base dia being a bit over what you expected could have been from a recut also.

FWIW,,the idea of the Ackley Improved cartridges,,and there's just about one for every commercial round of the era,,was to be able to use the commercial parent round in the A/I chamber with complete safety. That would leave you with a fireformed A/I case for reloading.
All working nicely in a PO Ackley worked over rifle of course or one done to his specs as claimed.

just my thoughts.