USQMC does mean US Quarter Master Corp, It would also mean the horn was purchased after 1912 and before 1962, when the USQMC became the Army Materiel Command. USQMC was stamped on horns that were purchased for general use throughout the US Army. I had a 1941 Holton dbl bell that had USQMC on it.
I believe horns that were purchased for the Washington DC bands most likely had their own stampings.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band