US stamps with capital letter A designation?

When the U.S. Post Office is trying to get a rate hike approved and wants to be prepared with new stamps, then instead of printing the amount of the postage, the Post Office prints a letter to designate the value.

The "A" stamp was worth 15 cents. It had a stylized eagle, and the Words "US Postage".

See the link "What Stamps Are Worth" for other non-denominated stamp values.