I think it was maybe 10 years ago or more, that a bunch of well worn Mausers like that were imported from China. Apparently many had bad bores, and there are still people relining the bores for a moderate sum. Might have been one that someone bought from that Chinese lot, and just stuck back without doing anything with it? I don't think they were outrageously expensive at the time, as they were mostly probably not in shooting condition after 60 years of hard Chinese usage. Now of course they are scarce again.
Your gun is like your nose, it is just wrong for someone else to pick it for you!