Subject to seeing any proofs on the barrel flats, there is a better than even chance that it was lined in the US. Had it been done in Germany, it should carry a "repair" proof, and new caliber designation, etc. I believe a chamber cast is warranted; failing that, you could at least fire a 8.15x46R and see what the case looks like. In my mind, the MM on the barrel is why I'm not certain it was lined in the US. That is the mark of Max Moeller of Zella-Mehlis, as opined by Raimey and Suhl makers(Sauer) didn't normally use their products. (However retailers often used them.) If an 8.15x46R case fired in the rifle keeps the correct shape, I can agree it is German work, because an American would have used an available caliber.
Mike
I had thought about the possibility of the lining being done in the US, but then I would have expected something like 32-40. It certainly seems to have been relined several decades ago.
I'll try to fit an 8.15x46R case first, then maybe fire form a case to the chamber to see what the exact measurements are. I'm kind of surprised that both this rifle and the drilling have the crown B and crown U proof marks but no markings indicating bullet weight or powder charge like my M88 sporters do.
