Here's some threads about these markings... Most of this has been discussed at some point here, easy to find.
https://www.milsurps.com/showthread.php?t=31874
https://www.milsurps.com/showthread.php?t=36348
https://www.milsurps.com/showthread.php?t=37452
https://www.milsurps.com/showthread.php?t=26508