This should be posted on the main forum rather than in the auction section, but this is the information you're looking for.
http://www.oldcardboard.com/enews/20.../enews74.htm#2
There are also blank backs which are more common than Red Cross but less common than all of the others. I believe almost all of them were hand-cut from a single sheet and broken up for individual auctions on eBay about ten years ago.