catalog/reference for Ryukyu postal stationery?

I'm always amazed by the breadth and depth of knowledge on this cozy little board, so I thought I would ask here first.

Is there a reference or catalog for Ryukyu Islands postal stationery?   This would be during the period of US possession, and probably beyond what the Scott US Specialized catalog lists.

In using the Googles, I was delighted to learn about the Ryukyu Philatelic Specialist Society, and while they seem to have amazing depth in their specialist publications on stamps, I don't see anything about postal stationery. 

And I see something on a specialist dealer's site that I'm pretty sure references a UPSS (United Postal Stationery Society) catalog number, but I don't see that publication available on the UPSS website.   Does anybody know if the Ryukyus is included in their US catalog for postal cards?

Any other thoughts, suggestions, leads?

Thanks in advance for anyone's help!

-- Dave