Japanese Primer Part III - Japanese Naming Conventions - Navy Short Designation System
So what do the letter and numbers mean that are associated with Japanese Navy aircraft? Actually, it's really quite simple.
The short designation was a combination of letters and numbers that helped to designate the type of aircraft, the model, manufacturer etc.
The first Capital letter indicated the function of the aircraft.
The first number indicates how many different model aircraft of this function have been made. For example, the A6M2 was the sixth in the Carrier Fighter series.
The second letter indicates the company that was responsible for the original design of the aircraft. The company designations were as follows:
Example: The D3A1 was a Carrier bomber (type D), the third in the carrier bomber series, made by Aichi.
The final number indicates the model of the aircraft. This number changes with each major change (engine, airframe etc), a small letter such as the "c" in A6M5c was used to designate a minor change in the aircraft. These minor changes were generally related to armament and weaponry.
Additional letters were added to the end of some aircraft. The -K designation in aircraft such as the A6M2-K meant that the aircraft was modified to be a trainer aircraft.