1911 thru 1917 70 ton H21a "quad" hoppers were introduced for coal service. During 1922 and 1923 all early H21 hoppers were converted to H21a's by replacing the 50 ton trucks with 70 ton trucks equipping them for coal service and bringing the total to 36,000 H21a hopper cars. The original 1909 thru 1930 quad hoppers were built with the "old style" double door hopper. During the 1930's and early 1940's all old style double-door hoppers were replaced with the "saw tooth" hopper, increasing capacity by 39 to 40 cubic feet making them easier to maintain.
Circle Keystone Lettering was applied starting in Jan 1930
Shadow Keystone Lettering was applied starting in June 1954
Plain Keystone (black car) was applied starting in July 1956
PRR Initials only starting in early 1960's