Fred,
They were called the I-400 Class submarine. They carried three complete Seiran divebombers and parts for a fourth. There really is a shortage of good information about them, especially plans and drawings. The only remaining item of an I-400 class sub is one of the bombers which is being restored at the Smithsonian in Washington. All the subs, and I believe only three were completed, were scuttled.
You can find some info at the following sight:
www.combinedfleet.com/kaigun.htm.
Frank