Cougar;
For all things IJN I rely on the Combined Fleet website. I’ve “communicated” with some of the guys who contribute there and they are really into the Nihon Kaigun (as the Japanese called their navy). They do know their stuff.
Try this link on the Unyo/Yawata class:
http://www.combinedfleet.com/ships/taiyo
It looks like Yawata Maru became Unyo, but the conversion wasn’t complete until sometime in 1942. US intelligence must have caught wind of the conversion of this and other liners and classed them all upon Yawata, without yet knowing the names to be given to the new ships. Or they did and didn’t want to compromise it in their press releases. So my take is that any liner converted to an escort carrier became lumped into the “Yawata class.” And it appears there were only three of them.
What always baffled me was that the Japanese hit upon a very effective aircraft carrier design in the “crane sisters:” Shokaku and Zuikaku. They were fast, modern and carried more planes than any other Japanese carrier. It would have made sense to put the design into as much mass production as they could. But they didn’t. It would be like the USA building two Essex class carriers instead of 24.
They never followed up on TAIHO, either, building just the one.
I use combinedfleet.com too, but I’ve never contacted them. Interesting to know.
I’d be interested to see the naval intelligence that led to this classification.