Aloha All,
The Japanese Naval aviation units made changes in tail codes about once a year for security and deception. This became more often as the war progressed.
The Japanese research on these changes continue. The URL below gives a basic understanding for MANY, but no where near ALL, IJN aviation units.
This may serve as a beginners guide to get you up to speed on the subject of our "afflictions".
Cheers,
David Aiken
http://www.warbirds.jp/text/kigo.htm