GWB is absolutely correct. UMC began the practice of using an impressed U on primers in the early to mid 1890’s to denote a cartridge loaded with semi-smokeless or smokeless powder. Usually on cartridges that formerly were strictly black powder and had “graduated” to smokeless. With cartridges that were introduced as completely smokeless, like the .30 Army, the first primers used were un-marked brass. Later, this caliber and others were loaded with “improved” primers, copper cup, impressed U, and I believe this was done to denote the improved primer. REM-UMC continued this practice, but by the time they came into existence, all of this was sort of passé, and I believe REM-UMC did this to indicate that these cartridges were loaded at the factory and not by “outside parties”…???