You have the M59 Yugo SKS variant. The M56/66A1 was the first variant to have the muzzle brake and grenade launcher. Your Yugo SKS was one of the first ones made. It was not sporterized. The M59's are worth more because they have lower production numbers
You sure about that? I'm somewhat familiar with the earlier Yugo M59 PAP variants and much more so with the M59/66 variants (and their nearly identical night sight-equipped M59/66A1 variants, which Stocknup's appears to be). Cherry's out of Greensboro, NC was a highly-regarded Yugo SKS importer but I thought they brought in the "/66" variants, which is even import stamped on his gun. Many Yugo SKS importers did leave off the "A1" designation on rifles that did in fact have the night sights.
Yes I realize he doesn't have a grenade launching spigot on his, but a lot of folks over the years removed theirs as well to shorten the overall length. Stock's example also appears to have the FSB section where the ladder sight would attach. I don't think the earlier M59s had that.