not exactly milspec, but I have also seen custom laser engraved dust covers on AR platform stuff that has ".300 BLK" or "5.56 NATO" etched into both sides of the cover, in addition to whatever markings might be placed on the lower/upper receiver bodies.
there is a color coding system for ammunition:
https://www.bulletpicker.com/pdf/MIL-STD-709C.pdf [PDF]
indicating special function, such as armor piercing; incindiary; fragmentary HE etc.
it is also a popular option to have receivers colorized or stamped. alot of magazines are polymer and could easily be colorized, and detailed, for instance with a large pictogram of the intended ammunition.
this is also on a little more of an angle, a reason to not let that combination of, persons you dont trust; .300 blk; and 5.56 chambers to occur.