Numbered name; Category 'A' air group (甲航空隊, Kō Kōkūtai) as combatant unit. Example 12th Kōkūtai is Training (bomber) group. 121st Kōkūtai is reconnaissance aircraft group. 762nd Kōkūtai is land-based torpedo bomber group. 1081st Kōkūtai is military airlift group. Regulation table Place name; Category 'A' air group as training unit, evaluation unit. Example Atsugi Kōkūtai Kasumigaura Kōkūtai Takuma Kōkūtai Yokosuka Kōkutai Region name; Category 'B' air group (乙航空隊, Otsu Kōkūtai) as air base guard unit. Example Kantō Kōkutai Mariana Kōkutai Hitō Kōkutai Squadron identification And after 1 March 1944 Regulation table Naval Aircraft identification System The IJN had, at the beginning of the Pacific War, three aircraft designation systems: The Experimental Shi （試）numbers, the Type numbering system and an aircraft designation system broadly similar to that used by the U.S. Navy from 1922 until 1962. Each new design was first given an experimental Shi number, based upon the current Japanese imperial year of reign. The Mitsubishi Zero so started its career as Navy Experimental 12-Shi Carrier Fighter （海軍十二試艦上戦闘機）. Upon entering production the aircraft was given a Type number. The 'Zero' was so fully known as Navy Type 0 Carrier Fighter （海軍零式艦上戦闘機）, as the Zero was accepted in 1940, or 2600 in the Japanese calendar. The aircraft was also given a "short designation" consisting of a group of Roman letters and numbers. The first letter (sometimes two) indicated the basic type or purpose of the aircraft. Second came a series number indicating the number of major sub-types produced for that type of aircraft. (Unlike USN practice, the digit "1" was not ignored in this system and was included.) Third was the second letter which was the manufacturer's code, and included some non-Japanese companies. (G4M designated attack bomber (G), the fourth in the Navy's sequence, designed or produced by Mitsubishi, while G5N would be the next attack bomber in sequence, built by Nakajima.) Fourth was a number indicating the version of the aircraft. The first production version of the 'Zero' thus became A6M1. Further minor changes were indicated by adding letters after the subtype number as in the Type/Model scheme above. The first two letters and the series number remained the same for the service life of each design. In a few cases, when the designed role of an aircraft changed, the new use was indicated by adding a dash and a second type letter to the end of the existing short designation