>Confusing the matter further, still other items are just numbered in
>sequence. Naval aircraft, in particular, got sequential numbers
>starting right around the time of the transition from biplanes to
>monoplanes, which is why the Zero is called the Zero. The first
>models of many naval aircraft were called "Type 0", including the
>Mitsubishi A6M ("Type Zero Naval Carrier Fighter"), and for some
>reason the A6M came to be known as the Zero by Western forces (where,
>for instance, the entirely unrelated Aichi E13A Type Zero
>Reconnaissance Seaplane didn't).

Well, according to Wikipedia (so take this for what it's worth, but pedantic issues about military hardware tends to be the sort of thing it's good at), the Mitsubishi Zero was also named for the Imperial Year it went into service, 2600 (i.e. 1940).

I gather from Osamu Tezuka's Adolf that Imperial Year 2600 was a big deal in Japan, combining a turn of the century with nationalism. So a lot of stuff got named after it, much like our wave of "millennium" stuff 15 years ago.