The rule on Mausers is there should be a number inside the side plate and it should be a number related to the first 2 digits of the serial number. It might be the same digits, or the first two digits plus 1.
It looks to me like you have a "5", which would be the first two digits plus 1.
The number "02" on the outside also looks correct to me. So overall I'd vote this is the original side plate to the pistol
Always hard to tell from photos, but I would estimate the pistol has been re-blued. The wear pattern, mottling and texture of the blued surfaces just don't look original.
All that said, I'm hardly an expert and will read with interest what others say. It is a very handsome pistol. Re: G.T., yeah I have "worked with" him to solve several issues on pistols - he fixes it, I agree with his fix, and behold "we" have it fixed.
