Jason.... 75 is the number you would want to see inside. From roughly 1939 forward Mauser stamped a number one more than the first two digits of the full serial number on the inside of the side plate on most of their military production.
IMO that it the original side plate for the gun.
Thanks for posting. The goal here and on other forums is to learn as we go. Every new gun is part of the process.
I would be pleased to have yours in my collection.
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dave
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