Fairly common I think and don't think bubba can be blamed on this one, I've seen the 4 digit serial stamped on either side of various Romanian stocks, in both horizontal and vertical orientation. I think it's field armour work...but, have no proof.
My '58 has the 4 digit stamp on the right side and the normal factory stamp on the left side... both numbers match the receiver. My reasoning for field work, is the only non matching item is the gas tube and piston, nothing else has been changed or like had a major refurb, and the factory stock number is kind of hard to read.