At Phosphorus32's suggestion, I was successful in getting a query to Branko Bogdanovic about the Long Barrelled Yugoslavs; initial results are here:
https://forums.gunboards.com/showthread.php?936521-SKS-Long-Barrel-wow&p=9796611#post9796611
Looks like the total number of these is about 200 rifles! Also, the suggestions of many here looks like they were right on the money for where these fall in sequence. As I read the referenced post, these were indeed prototypes for the M59, and not interim/preperatory to the 59/66 series. Now, where does that put 50456, C44-140, etc.? Anyway, a fun week for SKS info!
We have some 30 LBs on the list and not a single one of them goes above serial 100. The other '100' must be the odd-duck letter prefixed ones with a much higher serial.... Likely built at a later date in tandem with the C series.
These are the oddballs
C36439 1.75" with standard fsb
C44140 1.75" with standard fsb
50456 1.75" with standard fsb and chrome bore.
Reading this again... I realized the 520mm is standard length. So there were 100 standard length rifles and 100 LBs. Notice how he States "long barrel" after the 542mm length.
Looks to me he is stating one was 520 long and the other was 542 "long barrel"
LC, I blew right past that difference without being near enough to any of them to take measurements. Hopefully, more information will be forthcoming; Nothernug mentioned in a PM that Mr. Bogdanovic may be publishing more work soon in the U.S. What that may be is unknown.
I wonder if there are any standard length rifles that have been noted with 1. A windage-adjustable front sight base, and/or 2. Very low, three digit (or less) numbers?
Another re-read of the post on Gunboards: the difference between the standard length and Long Barrel length would be 0.87 inches, correct? Until this new information, the commonly noted/accepted length beyond standard has been listed as approximately 1.5 and 2.0 inches. I'll post the lengths of barrels to which I have access once I return, but can anyone point to posts that may compare examples of rifles that have both of these lengths?
The standard M59 barrel is 520mm or 20.4724". My LB #080 measures 559mm or 22.0079".
So, I think your guy has the LB length wrong, and the 520 is just the normal length. Makes sense... they test some normal length and some LBs. thumb1