The first cover (Hell's Half Acre, WY) is (as the rubber stamp suggests) a "First Day of Mailing" meaning that it is the first day of nationwide release of that stamp (July 11, 1940) and someone prepared these covers as an interesting local topic for the Wyoming Statehood Stamp (Scott #897). The actual first day of issue of that stamp was in Cheyenne, Wyoming on July 10, 1940.
Interestingly, "Hell's Half Acre, WY" is a discontinued post office that was only in existence for 19 years (from 1940-1959). A quick internet search suggests the area is virtually abandoned today as it used to have a motel, diner, camp site, gift shop (and presumably a post office) back in its day but has been closed down, gated off, and the motel demolished many years ago.
It's about 40 miles west of Casper, Wyoming and is more of a curiosity of rock formations, etc., that led to the name. Since there is no population there, it sounds like it was probably nothing more than a "tourist trap" in the middle of nowhere that eventually went bankrupt.
http://en.wikipedia.org/wiki/Hell's...re_(Wyoming)