These were 40-foot box cars of which the PRR had over 3,500. They were based on the 1918 WWI-era design established by the United States Railway Administration (USRA). These were steel-framed box cars with wooden body structure and metal external braces. The cars lasted until the early 1950s, but they were rapidly replaced by the all-metal rebuilt X26c cars. I took these photos at the Railroad Museum of Pennsylvania in August of 2013.
A close-up of the coupler, the Carmer uncoupling lever, the brake airhose, the poling pockets, and the various grab irons.