James Dick asked:
"The statement of 'From the mid-thirties production years, there were around 7300 in service, more than many roads’ entire fleets;' is applied how, by a mid 1950's modeler like me? (1953)
I am not taking a dig at anyone, or the PRR group for that matter - just looking for enlightenment."
Why does this matter? Tim Gilbert's body of work indicated that general service boxcars (i.e., not equipped with special equipment such as racks) tended to roam nationally without regional limitations, so large numbers of a given roads' boxcars would impact your piece of railroad, even if it's far away from the home road. 7300 single door Class X31A boxcars matter - maybe not enough to buy bunches of the Rapido model, but you can certainly justify one.
As for lettering, 1953 is easy. Circle Keystone, as the first Shadow Keystone scheme wasn't adopted until 1954. Jerry Britton posted a summary of Brady McGuire's seminal article on PRR boxcar lettering from the Summer 1988 issue of The Keystone.