Traco Transparencies?

[swarmee]

I just picked up two Traco (Traver Corporation) Transparencies from the 1950s on COMC from a seller who discounted them to a good price.

http://www.comc.com/Cards/Baseball/1...encies,sr,i100

I hadn't seen these before, but it looks like there's a Dodgers one on eBay and a Cardinals recently sold. Not much on the Internet about them, and a search here didn't reveal anything.

Questions about this issue for those that might know:
1) Did these come in a pack or how were they sold?
2) Is there a checklist out there?
3) What is a standard price for this issue?
4) How rare are they? Since they were intended to be used and disposed of, I'd guess there aren't too many survivors since I can only find 2x Orioles, 2x Browns (I bought one of each at $8.25), the Cardinals, and Dodgers online.
5) Does anyone else collect them? I like the graphics, they definitely say "50s!" to me.