Sure hope there's a little popcorn left in y'alls buckets.
Bad things happen when I start thinking so I'm likely to just embarrass myself but a thought occurred to me about this numbering system. No doubt there is a specific purpose, but here's the deal. (actually first of 3 including BBs "bow")
The "thread pitch" (or "space" between each thread) is 1.5mm. That means for every revolution of the barrel, it moves 1.5mm, or .059". So divide that .059" by 36 numbers, and that means they inspected/marked them with difference between each number at just .00163888". Wow, one and a half thousandths of an inch difference?!
Type of material can enter the picture here, but varying amounts of "crush" are accounted for when machining and measuring, in some cases the thread shank length, but more often and more importantly, the headspace and any "indexing" involved. BBs contention "barrel bore bow" might have been involved is highly doubtful, as this would likely only apply to guys at a bench with their ammo being kept at a constant temp in a cooler, until they load and shoot.
At Alpha Arms, we would machine the barrel, allowing that .002" of "crush" would occur with the torquing of the barrel to receiver, meaning chamber was .002" too deep, until barrel was actually screwed on. So far, all these facts fly in the face of inspecting and marking every .0016" difference. If it was only the numbers 0,2,4,6 etc, and .003" and 20 degrees difference between them, then it would seem a little more logical.
And to add, old Springfield 1903's must have been manufactured with at least .005" crush, as they are torqued on the receivers tighter than any other gun we've ever worked on. In some cases, the "take-off" barrel has to be machined in front of the receiver, just to relieve pressure and get them apart.
(3rd deal) Now we know 'Smiths are all crazy... but got an old retired 'Smith buddy that claims he has a video of Chinese SKS manufacturing, with English speaking voice giving descriptions, and while I'll reserve final "say" until it's been viewed, he contends it shows a round barrel being screwed on to the receiver, then both mounted in a mill vise and extractor cut is very next operation. (important fact...this removes the need to install barrel, mark for extractor cut, remove barrel and machine, then re-install the barrel.) That's always been our method as there doesn't appear to be room for normal machining operation that would do this without machining the receiver a little. (sure never seen that)
It's actually a 100 mi drive for me to go see the old AK/FN/HK/AR/Suppressor manufacturer/dealer buddy from the old days, (and he has cats) but I'll try to get a copy of it asap. May even ask him to snail mail it to me as he's the most 'puter illiterate 70 yr old I know.
With "pinned barrel" SKSs, I can see barrels being manufactured complete with all attachments already on it before attaching to the receiver, but threads muddy up the water. If the video shows machining of the extractor cut after barrel is screwed on, it only makes sense they would also then drill gas port hole, machine top and bottom side flats, then install all 4 barrel attachments.
But that leaves us asking...then what are the numbers for!? And likely has good old LC chomping at the bit to come back over and kick my arse! Yes, he was here and very well prepared for the testing, (kudos dude!) and the barrels were swapped and both indexed near bottom dead center, instead of top dead center.
One last thing today...Having removed quite a number of barrels from SKSs, I can say the amount of torque required to remove them varies between guns, even in a group with the same country of origin. And if numbering system was intended to match indexing of threads, the amount of torque used to "set" the barrel, should have been something they paid attention to. So the only thing that would cause us to be fully on board, would be if it was required to drill the gas port hole before chrome lining the barrel, and index numbers were just to get gas port at top center. Then it would all make sense.