The Rem-UMC headstamp (on all calibers) first appeared 1911-1912, or maybe quite a bit later, depending on when the UMC headstamp bunting dies wore out.  The same sort of parts replacement process probably dictated when the 30 US Army headstamp last appeared.  My guess is the late 1950s or early 1960s.   I have a 1956 Stoeger catalog with an illustration of a 30/40 Krag cartridge with a Peters 30 U.S.G. headstamp.

If you could show photos of the boxes, and maybe the cartridges showing the bullets, we might be able to come up with a more accurate guess.