A little bit of history recited from the Scott Specialized Catalog of US Stamps concerning the above specimen:
1. The stamps were available at five test sites in the Southern Maryland, Miami, Oklahoma City, Detroit and Santa Ana, CA, divisions. They were produced for use in ECA GARD Postage and Mailing Centers (PMC) machines which can produce denominations from 1-cent to $99.99.
2. The Scott Catalog for #CVP31 lists a 29-cent denomination as that was the current first class rate and was the only value available through the Philatelic Sales Division.
3. The most common denomination other than the 29-cent will probably be the 1-cent (catalog value 20 cents) as these were made in quantity, by collectors and dealers, between plate number strips.
4. Later the machines were adjusted to provide only 19-cent (postcard rate) and higher value stamps.
See Figure 3 at this link as to the type of machine that dispensed these stamps:
http://www.linns.com/howto/refreshe...rcourse.aspx