From 1956 to 1964, the Philadelphia Mint inadvertently struck some circulation strike Washington quarters with a reverse design intended for proofs only.
All regular circulation strike Washington quarters from 1956 to 1964 will have a reverse design displaying the letters ES of STATES very close to each other and almost touching. A 1957 proof Washington quarter design is different, since it will display the letters ES of STATES separated from one another.
Consequently, any circulation strike Washington quarter from 1956 to 1964 which displays a proof design is referred to as a Type B Washington quarter. The 1957 Type B reverse is probably hundreds of times scarcer than regular 1957 Washington quarters. Surprisingly, the 1957 Type B Quarter is the most easily obtainable of all the nine different Type B silver Washington quarter varieties.
It is very difficult to determine which Type B Washington quarters are ultimately the scarcest or the most common from all nine different dates. As time goes by, the PCGS Population Report should be a very good indicator on which dates with the Type B reverse are actually the most common and the scarcest of all.