The H21 subclass "a" hopper cars were built between 1915 and 1917. These hoppers were predominantly 70-ton coal carriers. By the time WW II came around there were over 38,000 of them. By 1951 the Pennsylvania Railroad had just under 32,000 of the subclass "a" H21 hoppers. They lasted well into the 1970's. The PRR Railfan web site was the source for the prototype photo of #194906. That same web site also has a photo of #177831. The book "Pennsylvania Railroad Steel Open Hopper Cars", 2000, Highlands Station, Inc., ISBN 0-9655365-4-8, starting with page 32 is another great reference.