Hello, like the American Units the lettering on the fuselage of an aircraft like GM/D was so you could know what Squadron it was with, if you have book/s on RAF aircraft look for the same letters "GM" on other P-40's and the photo may tell you what unit it is, or try a RAF site and see if it is shown there. the letter "D" would be for the individual aircraft "call letter" so someone in the same unit could tell them hey "D" you have a Zero on your 6 !
I hope this helps you some!
Johnny
WeBeEmblems@aol.com