Saw someone posted on Gunboards regarding a refurb letter gun. The person thinks the stock is the original, but the 3rd pic indicates the stock may be a later replacement style? Difference at upper corner of the buttstock?
https://forums.gunboards.com/showthread.php?1170771-What-Are-Russian-SKS-Letter-Series-Guns-Going-For-These-Days
If there is any truth to the notion that the Tula ceased production of the sks in 1955 and the letter guns were produced at various smaller facilities, it's possible that variation in the butt plate is just a trait of the facility where is was produced (or assembled). In that scenario it could very well be an original stock, and not a replacement.