I am far from a PRR guru, but I do have access to Ben Hom’s description of lettering for X29 boxcars.  I don’t know of my own knowledge, but I would be very surprised if Ben were in error.  His listing:

Circle K 1/30

SK1a       2/54  “Calendar style” numerals

SK1b      8/54  Serif-style numerals

SK2a     11/54  Serif style PRR and numerals

SK2b     11/57  Plain style PRR and numerals

PK            6/60

Where SK is Shadow Keystone and PK is Plain Keystone, and the dates given are the inaugural dates of each scheme.  Hope this helps.  –Ed Shoben