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In 2015 I wrote five posts on stamps related to Alabama. You can find links to them here: Part 1, Part 2, Part 3, Part 4, Part 5.
More about U.S. stamps and postal history can be found here.
The new stamp will be issued on February 23 at Constitution Hall in Huntsville. The photo was taken at Cheaha State Park by Joe Miller.
The post office issued this stamp in 1969 for the sesquicentennial on August 2, 1969, also in Huntsville. Back in April 2018 I wrote a post on the sesquicentennial activities in 1969.
In 1919 the Alabama Centennial Commission planned various activities to celebrate the milestone, but issuance of this half dollar was not one of them. The coin was finally struck by the U.S. Mint in 1921; follow the link to get the details of this convoluted story. The obverse of the coin features busts of William Wyatt, governor in 1819, and Thomas Kilby in 1919.
Wikipedia also has a long entry on this coin.