There are some aircraft designations that are interesting, confusing, or funny.
Here are my first two choices;
U-2 - Seeing that the U-1 was the DeHavilland of Canada DHC-3 Otter and the U-3 was the Cessna 310. The U-2 was definitely a red herring of a designation.
USMC C-119's - The Marines flew the C-119 in the 1950's and under the old Naval designation system, it was a R4Q. Honest!!
R - For a Transport, Q for Fairchild, the 4 meant it was the fourth transport Fairchild has built for the Navy/Marines/Coast Guard.
And, of course, the improved version was the R4Q-2.
Can anyone else come up with some interesting military designations?