I ran across another interesting variant this week. Probably should have bought it, but he wanted $400
It was a 1951 Russian (date on stock), refurbished. The receiver cover had no Arsenal marking and no date. It had only a very faint refurb mark on top and the correct serial number on the back (of the receiver cover). I say it was interesting because all other "no dates" I've heard about were 1955-56, but this one carried a '51 date on the hardwood stock.
This would seem to suggest that this rifle was made in 1951 with a "no date and no Arsenal mark" receiver cover. The only other answer would possibly be that a new unmarked receiver cover was installed and new matching serial numbers were stamped on it. That would seem to be most likely.
Also, the receiver cover itself was not a polished blue like mine, but more of a grainy surface blue, like some Chinese I've seen.
I'm beginning to think that in the later stages of production (1956?) they were using up spare parts on refurbs and new guns without bothering to go to the trouble to stamp the top of the receiver covers.