I may be wrong, but I always thought it had to do with the different naming conventions of the IJA and the IJN. Please correct me if I am wrong, but early in WW2, did not the IJ Navy engines have names by which the engine was commonly known. Didn't the Army use their own Ha number and/or a formal designation. Later in WW2 the IJA and IJN developed an "integrated" unified numbering system. For example the Japanese Navy called the engine the Honor 11, The Japanese Army called the same engine the Ha45, or the Type 4 1850hp engine. The late unified (integrated) system referred to the engine as the Ha45-11. I don't remember when the unified numbering started. 1943?