I have a Pennsylvania Railroad document titled Chicago Division Symbol System of Car Placement. This book has instructions, interchange schedules, and a list of all the industry tracks in the PRR Chicago Division. Among the text are instructions for marking the cars. I have the 1/1/52 revision and the 3/1/37 revision of this book. The following text appears int he '52 revision but not the '37 revision.
At dispatchment yards cars are given originating terminal inspection. Inspectors mark such cars on the Pennsylvania Railroad in yellow crayon. Foreign lines use crayon of a different color.
Each railroad operating within the Chicago Interchange District have assigned pool numbers; that of the Pennsylvania Railroad being 36.
After completion of inspection the car is marked with car inspectors pool mark. These pool marks will be located on both sides of the cars near the corner, as follows:
"CH" is a symbol for Chicago, "25" is the car inspectors personal number. On the next line the number of the receiving road will be at the upper left, and that of the delivering road at the upper right. Thus "36-2" indicates car was received by the Pennsylvania from the Belt Ry. The number at the lower left indicates the month and the number at the lower right, the date of interchange. Thus "7-20" indicates a car was received July 20th. The letter "I" being a yard symbol, in this instance-Colehour.
M -59th Street.
ASSIGNED INTERCHANGE NUMBERS
2. Belt Ry.
Now, I am on the hunt for photos of cars with marks in this format.