Most probably know this; but to my credit i didn't; for anyone interested Lets take the IJN A6M2c Type 0 Model 21 A stands for Carrier Fighter; B Torpedo //Level Bomber; D Dive Bomber, so on 6 stands for the sixth such type ordered M stands for Manufacturer 2 stands for a major modification c stands for a minor modification TYPE 0; The navy used the last single number of the year in the Koki dating system; and it had to be year of planes accept use; 1940 is 2600 Japanese. Model 21; an increase in the first number represents a change in the air frame;; in the second number an engine change. A A6M5c for simplicity could be called Model 52C ; Navy - Type 0 - Carrier Fighter - Model 52C Add a K and the plane has a different than originally envisioned use. Read some IJA aircraft nomenclature got a head ache. By 1943 both the Army and Navy for security did changed things a bit; again you have to be Stephen Hawking to know of that. Hope it helped and i got it all right