Ron Guth: The Standing Liberty Quarter Dollar was designed by Hermon A. MacNeil. The initial design (Type 1) included a bare-breasted Liberty, which was redesigned quickly. Instead of covering Liberty's breast with the same flowing material of the rest of her dress, the designer clothed her in a coat of mail (Type 2). The reverse design was also re-worked in 1917; the eagle was moved more to the center of the coin and three of the thirteen stars that used to be on the sides of the reverse were now placed beneath the majestic bird.
Well struck examples are the most desirable and much attention is focused on Liberty's head. So-called "Full Head" examples (those with complete details) often bring considerable premiums over poorly struck examples, but attention should also be paid to the rivets on the shield. Any coin with a Full Head and full rivets is a true prize. Mintmarks on this type ("D" for Denver or "S" for San Francisco) appear on the obverse just to the left of, and slightly above, the date. The small M that appears to the right of the date is the designer's initial.
The "key" to this short-lived series is the 1916, thanks to its low mintage of only 52,000 coins.