Bill Welch wrote:

Did the Pennsy have an official or de facto designation for the common

underframe they used with their K7, R7, and X23/24 group of cars?

Bill,
According to the trade press articles there is no such designation other
than it was a common design leading to all-steel car construction. The
article claims that shippers, traffic department, agreed to have the box
car also to be constructed from wood, but should conditions change the
sides could be easily replaced with steel sheathing. [I guess the war
emergency designs were not so original]
The underframes are described to be of the same type as used on
passenger cars where the load of the superstructure and the lading are
transferred to the centersill through the crossbearers or cantilevers
and the end sills. Apparently from the description the body bolsters did
not transfer any of the load.
Bob Witt
References:
Railway Gazette Mechanical Division vol. 88 no. 8 August 1914 p.419
[X25]Railway Age Gazette vol. 53 no. 14, October 4, 1912, p. 632 [X23,
R7, and K7]American Engineer, October 1912, p. 502Daily Raiway Age
Gazette June 13, 1914, p. 1412