I have Robert Liljestrand's and David Sweetland's book Pennsylvania Baggage and Mail Cars. I would have liked a little more detail in it, but it provided some good pictures of these cars.
Class B60 first appeared in 1906 and had the celestial roof. 360 cars were built and weighted about 103,000 lbs.
Class B60a had the round roof and had large end doors for auto shipments. They gave the impression only 16 were built. No date given on when first built. (ADDED: first built in 1913 per another web post.)
Class B60b first appeared in 1925 and had the round roof. 537 were built and weighted about 87,000 lbs. to 101,300 lbs. depending on the trucks used.
From the book, "By the early 1950s, two hundred express messenger cars were in service on the PRR." Looks like these cars were rebuilt from B60b cars over a period of several years in the 40s through the early 50s. These cars had 8 vents added to the roof for the messenger, plus a desk, lockers, wash room, water cooler, and a safe for the packages he/she was transporting.
All cars had a 40,000 lb. load limit.
The Keystones were part of the last paint scheme for these cars.
It would have been nice if the book gave more details on these cars. Apparently B60 cars were rebuilt into B60b cars, but no details on how many or when. A paragraph stated 40 cars were rebuilt in 1965 and got the Keystones, and these cars carried over to the Penn Central years.