The 1917-S on obverse is a key date among Walking Liberty Half Dollars that, many years ago, was the most rare and valuable prior to modern population reports, which show the 1919-D and 1921’s to be far scarcer. It is never a common date no matter the grade. Scarcity begins at the XF range and rarity in Mint State grades through MS66. The finest known grade is MS67, which is very rare.