After raking through my references, there was some Alternate Standard and Substitute Standard powders pressed into use in 1942 is the rushed build up during that year.
Now, most of that was rifle powder, and into M2 Ball (some was directed to training and gallery loads). But, since the powder could not be guaranteed to be 100% reliable to military performance, it was labeled for rifle use only, to divert it from being used as MG ammo, where performance was to a much higher standard (and the Browning MGs were sensitive to headspace issues).
As a guess, a "rifle only" label was stuck on a pistol ammo box. Whether that was an uninformed guess from finding the label on the floor or a wag looking to stickyfinger some pistol ammo is speculation at this point. Or, if a person with a butt-ton of boxes just had a spare label to hand 50 or 60 years ago is also just speculation.