I had thoughts about the rounded ends to the fore-end numbers. I seem to associate those stamps with the machine made stamps from the 70's. As a matter of interest, I have the original '51' of the S 51 'liberated' from the examiners bench at the H&H factory.
The sanding marks look original to me, just like it came from the factories before it was smoothed by touch. Some of the factory butts also have the distinct flat on top of the wrist too. It's quite common. The finish also looks like the H&H brunofixing too. They used this particular finish because it was a chemical finish that would affect the area that they'd worked on. That's the reason apparently, it's a bit patchy