In military service the headstamp bunters would have to be replaced annually to denote the change in the year.
That often required scrapping perfectly servicable bunters.
Except in 1944, when 1943's bunters were taken to the tool room and had the 3 ground off, leaving just an off-center 4.
This apparently wasn't done universally at every US cartridge manufacturer, and as the altered bunters wore out they were apparently replaced with ones with the complete year marking.