Yes. My '38 Tula PE Ex-Sniper has the same 'C' mark on the top of the forward portion of the receiver. All of the Tula PE and PEM round receiver snipers that I have ever seen have the same marking. It must be some kind of proof mark. It can be a little hard to see on rifles that were heavily buffed during the refurb process.
I saw this mark before, but I cn't tell if I saw it on an ex-sniper or on standard 91/30s.
During the last year I observed at least a couple of hundreds of refurbed 91/30s...I take notes every times I find or I look something intresting; I cheked my book but I didn't wrote anything about this mark.
Anyway, I will check a friend's ex-sniper a.s.a.p. and I will let you know if it has this mark.