In searching through the ORERs for another project I came across
USQX - the Q is for the Quartermaster Corps which was the Army's
supply arm in the early days. Later the Ordinance Corps was split
off the Quartermaster Corps. Ordinance supplies weapons, ammunition,
vehicles and POL (petroleum, oils & lubricants).

Besides USQX there is USSX and GPRX on tank cars back in the 1920s,
USNX (Navy) on tank cars, USOX on tank cars, there was also CWSX
during WWII.

USAX and DAFX (Army and Air Force respectively) did not come into use
until after WWII.

I have a few ORERs but certainly not a complete set so the dates
below are the first instance I found in the ORERs I have.

CWSX - April 1943
DAFX - July 1954
GPRX - March 1923
USAX - April 1950
USNX - January 1942
USOX - January 1942
USQX - December 1928
USSX - March 1923

Gene Green