Ron Guth: In 1917, the design of the Standing Liberty Quarter Dollar was modified. Liberty's bare breast was covered with a coat of chain mail and the reverse design was modified by moving the eagle more to the center of the coin and by placing three of the thirteen stars (that used to be on the sides of the reverse) beneath the majestic bird. Beginning in 1925, the dates on the coins were recessed, giving them more protection from wear. 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.
All date and mintmark combinations in this series are affordable, although dates such as 1921 and 1923-S are valuable even in low grades. The 1927-S is a condition-rarity that becomes quite elusive and valuable as it approaches the Mint State level.
The real "key" to this series is the classic 1918-S 8 over 7 overdate, a coin that is extremely desirable in all grades and which is exceedingly rare in Uncirculated condition.
No Proof Standing Liberty Quarter Dollars are known.