I was wondering if my SKS was actually built in '78, or was it just rearsenaled in '78 and renumbered as a '78 SKS?
My serial no. is O 609xxx, and it has all matching numbers. It's also got another number 224xx that's on ever single part that has the 609xxx serial number on except the stock. It's also on some parts like the firing pin and the rear sight as well that the 609xxx number is not on. There is also the initials "TK" on the bottom of the handguard gas tube, and also on the bottom of the rear sight.
The 224xx numbers look to have been put on by hand and not with a die set. There is also a 224xx number on the base of the barrell where it meets the receiver. At that location, the 224xx number looks to have been stamped with dies as the numbers look good.
The barrell has a slightly different color bluing to it than the receiver and the rest of the rifle. The bluing looks better on the barrell and the inside is very shiny with no visible pits. I'll go out on a limb and conclude that this is a replacement barrell.

Anyway, just what is that 224xx number?
The rifle also has phosphorus night sites front and rear, and a non-ported grenade launcher with the non-tapered ring mount.
Anyway, is it possible that the rifle was at one time a M59 and it was rebuilt in 1978 and renumbered?
Would a M59 that was converted into a M59/66 have kept it's original serial number?
Just curious.
edit: My stock has the 609xxx serial number on it. It also has two arsenal repairs on it. One on the outside of the stock, and one on the inside of the stock. These repairs had to have been made after 1978 unless the brand new guns came with these repairs on them.

Anyway, I think my SKS was probably made in '78 due to this. I'm still curious as to how we are sure that these guns are in fact made in the year that they are numbered. I'm also still curious about those other numbers. Could they be import numbers? Would the importer even take the bolt apart and mark the firing pin as well? If they went to all that trouble, they might as well strip them of cosmoline for us and charge more! :D
another edit: I guess they could have sanded the original number off of the stock and restamped it in '78 so maybe it could be a M59? Anyway, I don't know and it really doesn't matter one way or the other to me! :D