I'm a bit surprised these are collected as something special.
Toy town post office was a game sold in the same time period as the cards were issued. It's a pretty cool game, more of a play kit. There's the stamp of course, and a bunch of small envelopes and a bunch of tiny stamps and a post office window. The Spellman postal museum has one that was on display for a while. Here's on sold by Skinner.
http://www.skinnerinc.com/asp/fullca...707633&image=1
There was a different box too. I think this one is the one spellman has. If I recall it right the stamps are tiny replicas of US stamps from the late 1890's
http://www.flickr.com/photos/21728045@N08/5612066552/
I can easily picture kids using the cards as "postcards" so they're a bit cooler than the other rubber stampings we see. I'd really like to see one where they used the stamp from the kit.
Steve B