Ron
Yes, that letter and those numbers could be a unit mark to the 28th Field Artillery. This mark is not correct as to how regiment numbers & letters were supposed to be applied, but not every command followed orders to the "letter". Field Artillery unit marks are quite common on the LP-08 holsters.
The 28th Field Artillery was assigned to the 32ed Infantry Division.
Then again, that mark because it is not correct, might have nothing to do with Imperial unit marks.