In Jerry Kuhnhausen's shop manual on Mauser 91's through 98's, he advises strongly to make a chamber cast of EVERY new-to-you Mauser rifle before working on it, firing it, or letting it leave your shop.

His feeling is that so many of them have been re-chambered to other than original cartridges, and not been marked with their current chambering(s), that it is only prudent to do that. He feels that is especially important for licensed commercial gunsmiths because of the liability they may accrue if they work on or sell such a gun without absolutely positive chamber identification.

I'm not sure that frequency of odd chamberings matches my experience, but then I haven't had hundreds of Mausers. What is the strangest chambering you have found in a Mauser that was not marked as to the cartridge it was currently intended for?

I have an otherwise original Model B marked for 8 x 60 that is actually a 35 Whelen. It was rebored and rechambered by LaBounty but all of his markings are below the wood line. I knew it in advance but someone else might have been terribly disapointed at best.

I have not decided what to do if I choose to sell it. I think I'll have to re-mark it in a visible location before I can let it go. I guess it depends on how and who I sell it to.

I've only seen one and wish I'd taken photos of how the magazine was set up, but an M98 in 303/25. No markings anywhere to indicate its chambering.
Cheers...
Con

It was probably converted from the Mauser 303 slant-box magazine. They look like this: