I'm guessing it's a '51, cuz I'm not that lucky Assuming it's got scrubbed and force-matched numbers, it'll be hard to tell what year it is for sure, unless running-man drops by for a consult on the serial number prefix from his database
I'll be happy to help though I'm having trouble truly deciphering what the auction house actually means when they say "KHN19## = XH19##"
There are no X's I'm aware of in any SKS prefix in any year (yet). I've seen them in Izhevsk suffixes, but not in prefixes (yet) I suspect this one is indeed XH because the translation is correct based on how CAI did theirs: X = KH, H = N.
So assuming they got it right and it's XH, I have zero guns like that in the database. Can't help you there at all, sorry man!
I do have ФН prefix verified guns from '50, ШН prefix verified guns from '50, and УН prefix verified guns from '50. XH is alphabetically right in those blocks so it's entirely feasible though we know that alphabetic order is not necessarily something to be followed with the SKS45.
The feature set progression isn't much help from what I see in the photos, but I believe an XH prefixed gun from '50 may have a spring loaded firing pin since ФН & ШН both are, though УН prefixed guns have the late bolts and non spring loaded pins. So there is that to look forward to!
Tula stamping on the receiver cover top should be a 'modern' stamp, this gun is nowhere near the transitional '50 range:
In addition, if it is a late '50, it should have a:
45° gas block
short stamped receiver cover takedown lever
square ear solid bayo lug (no lightening cuts)
early tall receiver with matching RSB
tabbed gas tube takedown lever