Decklid
M
U
Shawn,
I don't know if you are aware of this, but the letters on the trunk lid are common for 1964 thru 1968.
Per the 1967 Mustang Body Assembly Manual (AM0016) pg 36, they are Ford part numbers - C5ZB-16D042-A for the "M", and C5ZB-16D042-B for the "U". (In case you locate the letters without retainers, each letter is retained with 2 ea 378362-S102.)
Jim
Hood
R
The hood uses a full size Ford part for the "R", C7AB-16606-C, service stock number is C7AZ-16606-C. (The letter is retained with 2 ea 378362-S102, same as the trunk.)