All the ones I can see are just the run of the mill process stamps that get added when the rifle is being built. The meaning of most of them has been lost to time, but basically they were thwacked on in a specific, unobtrusive position (like the bottom of the receiver) when a specific milling step was done or it gauged OK for moving onto the next assembly process or it was polished in preparation for another operation etc. I don't think those marks have ever been successfully trended in any meaningful way. I try to snap pics of them when I work on my guns just in case I am ever able to spot a pattern between them, but I never have. Then again, my collection is also much smaller than many here. I'd be very curious to see how, for example, a 1916 Oberndorf would stack up to a 1917 Oberndorf or a 1916 Spandau in that regard.
I will say that the position of the "7" you see repeated on a lot of the parts is extremely non-standard. I don't think I've ever seen a SN stamped on the underside of a bolt root, for example, and I've certainly never seen one on the inside of that gutter in the inside of the trigger guard or the back side of the mag floorplate. It's also on the wrong spot on the safety and missing from a bunch of parts that would normally have it. Military SNs had fairly standardized placement and were both visible and external (except for parts that ended up on the inside of the gun after assembly, e.g. trigger components and the firing pin). Plus as I mentioned before the apparent double serial on the firing pin.
But, you also have what look to be inspectors stamps on the cocking piece and the firing pin, which I don't think you would expect on a purpose built commercial gun.
The more I mull it over the more I keep coming back to this being scrubbed in an organized fashion by someone who was converting a bunch of military weapons to commercial. Slapping a unique number on the parts in an unobtrusive spot makes sense if you're doing this with a bunch of guns and want to make sure that parts that either came together or that you know work together stay together. That would also explain the oddities like the apparent inspection marks on a few parts or the double SN on the firing pin, a part buried far enough inside that they might not have bothered scrubbing it.
Really the only thing I can tell you for sure is that it was commercially proofed in Germany before 1939 and after ~1900 or so (whenever the crown/N proof was introduced, can't find it in my notes right now) and that it was proofed for jacketed bullets (that's what the St.m.G. means).
I'm strongly leaning towards it being built up between the wars out of either surplus parts (lots of factories had stuff left over when production shut down after the war) or a whole gun or guns that were broken down for the process. There was a booming industry in that in the inter-war period as gun companies needed something to do and there were a lot of military arms to dispose of. Gew98s converted to crappy bolt action shot guns used to be a thing, for example.
Some places did this in a more organized fashion than others. Mauser Oberndorf kept making sporting arms between WW1 and WW2, same as they had for decades before that. The old imperial arsenal at Danzig moved over to commercial arms for a while as well, even though they had been a government arsenal before Danzig became independent. The arsenal at Erfurt did as well, but the work tended to be much higher quality than what I'm seeing here. Engraving especially. But you also had a cottage industry of small firearms manufacturers and gunsmiths all across Germany - especially in SE Germany - and all those people needed to put food on the table too.
That's my current best guess. I'll defer to anyone who knows better.
edit: a lot of the work fairly screams small time gunsmith too. In particular I'm looking at the engraving on the receiver, the buttplate, and the checkering on the bolt handle. Whether that was how the gun was assembled or if it was done later (either in Germany or the US) I don't know.
