Still waiting to see my copy.
Some of the marks may indicate what line or shift was involved. As production of the bolts ramped up they had to be made on different lines. When a problem arose being able to ID which line was causing the problem would be important. Some marks are probably from different inspections and or inspectors.
Bolts are way more complicated than they look the locking lug, guide rib, and bolt handle are all indexed to so many degrees, besides all the linear measurements. Cam surfaces for cocking and on and on.

BR, I thought I was seeing a pattern in the SXX numbers with yours and my reports. If the SXX mark does mean anything it would be a late development to simplify rebuilds. Whig said earlier bolts had few if any marks.