Somtimes it helps to "see" what makes a ceratin Washington Quarter a key coin to the series. Below are a few graphs that visually demostrate what makes the keys and semi key coins of the series.
The graph below is a graph of all business strike Washington quarters from 1932 to 1963. Arranged in accending order by Year, then by mint location in order of P, D, S. This chart ends in 1963 because 1964's combined mintange is 6 times higher than 1963's. This distorts the scale of the chart so much that the chart scale become to distorted and the lower D and S mintages for the earlier issues become too small to see on the scale.
Business strikes from 1932 - 1963, Ordered by Mintage, Lowest to HighestFree Joomla Lightbox Gallery