"the last letter in the rubber-stamped marking indicates the nut width, as per the following code used on all the Fender instruments.
A = 1 1/2" (1,500")
B = 1 5/8" (1,625")
C = 1 3/4" (1,750")
D = 1 7/8" (1,875")
Back in the early 1960's, "B" was the standard size on the Stratocaster neck, "A", "C" and "D" were optional at an additional cost."
(fonte: The Fender Stratocaster , by A.R. Duchossoir)