To clear up some nomenclature, mP54 was a coach built without the electrical gear but upgradeable to be a full MU car. the MP54 was fully motorized.
There was no true P54 class according to PRR archives. Only mP54s that never got the upgrade to MU cars.
To make matters more confusing the lower case letters are only used as way to indicate that the 'm' is actually capitalized, but with a shorter text height.
For detailed information this is a good Link. You will note near the end of the thread that GG1 hauled troop trains did have D78 diner and a PB70 in the mix during WWII. Goes to show it really doesn't matter as it's your railroad.
I have mPB54 in 2 rail brass arriving tomorrow that will I will be converting to powered status and as a template to upgrade a set of 3R Lionel MP54s to 2 rail with improved details. Interesting cars indeed and not as easy to follow on the history for sure!
Getting into baggage, baggage mail, and other configurations makes the puzzle even more confusing.