Truth is... it`s not a big deal unless it`s convenient for the involved persons. How many M1 Garands are guaranteed 100% original and yet they remain collectable?
The marks on German Mauser parts are stamped to ID the inspectors with personal stamps so somebody gets hung if bad quality slips through the line and collectors thrive on those markings. As far as numbers match just the last few numbers of the serial number are on some parts that could have been added at any time.
Think about it, an all original is great if all original isn`t worn slap dab out, however, there were some all original Mosin Nagants that are in unissued condition, especially the later manufactured ones. I know, because I own one. I consider my Mosin Nagant a highly collectable...others may not!
If you have what some are calling a Frankenstein then ok because original worn parts could have been replaced by better ones. I have seen MN bolt head locking lugs sold in bulk as much as 30 items...some marked...some unmarked. So, out of the bunch there may be one or more with the right amount of steel to bring the headspace back into specs. If marked..ok, if unmarked then stamp numbers on it and then ok or don`t stamp number at all and still ok. Get the point? If you are wanting a safe shooting rifle the stamped numbers are not important.
BTW: You may have gotten the drink can shim advice from me or Gunplumber. I remember we had a similar answer for a budget headspace check. If everything you did with shim is what you say the Mosin Nagant GO gauge would have closed and NO GO would not have closed. You were not the asker. You have about .004" headspace which is factory setting. What about your firing pin height??
Also, have used the small hole gauges many times. They`re not as good as pin gauges but if you have a bore with .303"+ you have a really nice bore with very little wear. You are lucky to find a 91/30 with both headspace and bore correct. Great buy!