Could be the "main" components on orginial rifles were serialed when each rifle was build. The reciever and stock get numbered for tracking, the bolt and carrier, well at least the bolt. The bolt being pretty much custom fitted, due to headspace, sure others would interchange and fit, but that particular bolt fit perfect. Considering the bore variances on the crate placards, there may have been just as many variances in the machining of the reciever requiring numbered bolts. Im pretty sure, they didn't whip these out on a C-N-C machine. All it takes is a few thousanths of an inch to seperate a good head space rifle from a bad one.
The trigger, gas tube, piston, box mag may have been penciled at refurb, maybe at some point they opt'ed to start numbering all pieces, which is why some are blank. There is no practical reason for these pieces to need numbers, other than maybe tracking, these pieces are pretty much interchangable, not techincally fitted to the respective rifle, more like just "installed". And, you don't know how many trips to a refurb facility an individual rifle has been, was it just once, 5 or maybe 10 times.