I would say it is common for Oberndorfs to have a different "font" for the serial number on the barrel and the front ring so that doesn't suggest anything strange to me.
As for the patch at the rear of the tang, it is also common for Mausers stocks to chip out there. Some I have seen have been pretty sizable depending on the grain of the wood. I could see that a big chip could be patched in this way and the tang looks totally standard to me. Perhaps it could have had a very short based peep sight fit at one time right at the rear of the tang but it would have been a very short base at that...