Letters were used to differentiate between the first, second and so on editions of a particular body style. For example, a 1928-'29 Coupe is a 45A whereas a 1930-'31 Coupe is 45B. In other cases, letters were used to differentiate between different manufacturers of the same body style. For example, the 165 Fordor made by Briggs is a 165D, and the same model Fordor made by Murray is a 165C. An excellent reference book on this topic is Steve Plucker's " The 1928-1931 Ford Assembly Plants & Their Production of The Model A Ford Passenger Cars."