Thanks for the comments, everyone. I hate to keep posting these at random, but that's the way life is at present: hard to guess what any day will bring, now.
I've reached out several times to various people in Serbia/former Yugoslavia regarding these rifles, but it may be that not enough time has passed for it to be appropriate for discussion, at least for now. Phos32 and LooseCannon, have you had any luck in overseas contacts regarding the long barreled rifles? Browsing through old posts/websites/discussions, I keep coming across the name of Branko Bogdonovic (spelling?), but I've not found anything posted by him recently. I'd just like to know what these rifles really represent, as I love the design and have been a fan of the SKS since viewing old news articles about Vietnam as a kid, and the prevalence of the SKS in arming our enemies. I know that I'm prone to conjecture, but as for now, I can't wrap my head around numbers 50456 and C44-140 (and at least a couple of others with serial production receivers) and why they may have been made with the longer barrels, if "long barrels" were precursors to M59 production. Conversely, if numbers 031 and 048 were refurbished (as evidenced by the TRZ stock marks), when was this done? As I understand it, 031 came from the remaining items at the arsenal at Zastava, and 048 may have been an earlier import that could represent rifles that had seen combat in Yugoslavia's civil conflict. Anyway, I'm always hoping to learn new information, and look forward to your thoughts. As an aside, I've got a sleeper (literally sleeping in the safe pending cosmoline removal) of an Albanian-used Type 56 that I hope to post this week, if all the stars align-interesting "trench art" on the stock, and named to at least one user!