Mail Pouch
Barn advertising, central Ohio, by Ben Shahn, 1938.
I came across this while browsing Documenting America, the parent site of the Library of Congress exhibition mentioned below.
You can still play "Mail Pouch" in Ohio, although the signs are nowhere near as common as they used to be. Basically, all you need is a car full of people driving through the countryside, and the first one to see the sign shouts "Mail Pouch!" That's it. This is, after all, Ohio.
My parents were long-time friends of Ben and Bernarda Shahn (Bernarda died just last year at 101). They were neighbors in Roosevelt, NJ (which has an interesting history) right after WW II.