Ron Guth: Two types of Half Dollars were issued in 1839: the Capped Bust, Lettered Edge type and this, the Liberty Seated type. The latter design was late coming to the Half Dollar (it first appeared in 1836 on the Gobrecht Dollar, then the Half Dime and Dime, then the Quarter Dollar, and finally, the Half Dollar).
The first sub-type, known as No Drapery, refers to the lack of drapery below Liberty's left arm (the viewer's right). This was the original design, as intended, and it wasn't until later in the year that the design was tweaked by adding the eponymous drapery and changing the shape of the rock on the left side of the coin. Later "No Drapery" varieties in 1840 and 1845 were caused when over-polishing of the dies wiped-out the newly-added drapery!
This is a one-year year type and the first year of a rather lengthy series, thus it is the subject of great demand.