"The following is reprinted from "The Complete Guide To Certified Barber Coinage" by David & John Feigenbaum:
"The Barber quarter series consists of 74 regular issues, with no significant varieties. While it does not have a rarity equivalent to the 1894-S dime, it is a very challenging set with three well-established keys: 1896-S, 1901-S and 1913-S. Their very low mintages; and scarcity even in the lowest circulated grades, distinguish these three. In fact, the 1913-S quarter with a mintage of 40,000 has the distinction of being the lowest mintage (regular issue) silver coin of the 20th century.
The working hub (design) used in 1892 was modified during that first year to accommodate better stacking of the coins in a roll. Therefore, we have a type I and type II reverse for 1892 and type II from 1892-1900. In 1900, the hub was again modified slightly. These changes can all be distinguished by the relationship of the eagle’s left wing."
Sources and/or recommended reading:
"The Complete Guide To Certified Barber Coinage" by David & John Feigenbaum
Designer: Charles E. Barber
Diameter: 24.3 millimeters
Silver - 90%
Copper - 10%
Weight: 96 grains (6.3 grams)
Mint marks: The mint mark appears as a small letter just below the eagle's tail on the reverse, as follows: