
Now, the 18 inch Type 91 (marked in red) is the device equipped to all Japanese planes capable of carrying torpedos.
The 45cm Type 91 Mod series (marked in blue) is what I assume to be the upgrade path to these torpedos. According to a quick Google, the Type 91 modifcations consisted of some minor bodywork changes (with no real impact to the game) and changes to the warhead (important to the game).
This is an issue of mild importance, as the last Type 91(Mod 4) has a higher effect and penetration rating than the preceeding models, and by the time the Mod 4 is avalible (8/44), a Japanese player wants every torpedo hit to do as much damage as possible.
Is this a database error or was upgrading torpedo devices a a part of the game that the devs never got around to finishing? The inconsistancy in naming (the imperial to metric measurement switch) suggets the latter.