How did the Yugoslavians do it?
The only thing I can think of is they do it like Ben Murray does with the NOS 59/66 barrels on the market. Those barrels come with no flats no extractor cuts, and no gas port. He has to index and torque the barrel, machine the flats while installed, remove the barrel to cut the extractor, then adjust headspace where necessary.
Its worth noting, I bought one of those barrels myself and there are NO inspection stamps which you would think at the state of completion there would be.... so Im not sure if they were even finished.
It could be those barrels were not completed and Yugoslavia could have simply used an ink stick or pencil to mark the barrel and these numbers were later removed during the finishing process.
Also, Considering they only had TWO facilities doing rework on their guns, these numbers may have been redundant for them to stamp.
..... They COULD have found the physical numbering of the parts impractical since they were to be placed in a corresponding numbered bin.
One thing the Chinese mastered.... QC the part, dont waste time marking because the entire bin its going into is marked.