Hi Dick:
I have just re-read
Bill Lambert's book, "COMBAT REPORT". Ihave looked to see if I could find a reference to his identification letter. I have found the serial numbers of all his S.E.5a machines, but no letter, He was assigned to C Flight, which used: U, V, W, X, Y and Z. The Flight leader would have used the first letter, but not necessarily. Most C.O.'s usede the last letter, Z, however not always. I have looked through everything I could find on S.E.5a assignments and Flight letters. There is a possibility,and I don't have it, is the S.E.5 book by Sturdivant. Every S.E.5a is listed there, and it may have the information you seek.
Good luck,
Dan-San