A friend and '03 guy (a Chosin Reservoir Marine who actually carried an '03 in combat, by the way), has a standard SA B2 bolt also marked n.s. on the top of the bolt handle. This same bolt, interestingly, has a "4" stamped on the top of the gas ported extractor (top in as next to the extractor collar when the bolt is closed.)
May not be rare, but neither he nor I can recall having seen either B2 and n.s. on the same bolt, nor the number 4 on the extractor. Are these known markings?