Well I think I have this one figured out. It was made as a 12.7x70mm Schuler .. rimless aka 500 Jeffery. I can see where the plungers used to be in the extractor, the holes are now blanked and barely detectable. Somebody decided a rimmed version would extract better, and had it rechambered for a rimmed version based on 577 Nitro case shortened to 70mm, otherwise same profile as 500 Jeff. Form dies and loading dies from RCBS. Kicks like a mule I have to say. But I think it would be hell on elephants and buffalo.