Here is an Arsenal /26\ 3M series that also has some form of the Star 1 on the buttstock and what looks to be a faint star to the left of the serial number on the receiver. This rifle has been discussed on a few different forums and maybe you guys have discussed it as well. The markings are unusual and don’t fit the norm but nevertheless it contains some form of the marking being discussed here. The experts here may have more insight into this particular rifle.
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I'd be interested to know if anyone has seen an M21 with the Star 1 marking or other /26\ rifles for that matter.
I know nothing about SVDs, and arguably not very much about Chinese Type
54 56 SKSs, but I'll toss out what I'm seeing here. That stock has an original 1/4 tall x 1/10 wide lettering for the original 6 digit S/N stock stamp. It is not a great resolution photo an I can't tell if it matches the receiver S/N or not. It's possible that it does as I *think* I see a "31" as the last two numbers, but more interesting is the S/N restamp in the very tall but narrow lettering. The Chinese did not begin to use font even remotely that large until much later in production. You really start to notice the larger font in the 10 mil /26\ series when many other changes occurred.
This factory stock has numerals are equally wide as they are tall (roughly 3/8" x 3/8" whereas the ones in your image are at least the same height if not taller, but a bit narrower) and they have a full 5 digits (0 to 99k ) to be able to track 100k guns at a time, unlike the 4 digits of the restamp S/N you show which would only have been able to track 10k guns at a time.
Here's an interesting one I found in my records. 7 million /26\ (1962 manufacture) with a clearly replaced/refurbished stock (should have a side swivel on it). Unfortunately no clear photos of the right side of the stock, but very similar (I'd say the 1's & 2's look identical) font to the star 1 example you show.
I've never seen an M21 N
o marked with anything remotely like a star 1, though with so few known examples out there, that's probably not surprising.