Having trucks stenciled with the reporting mark and car number was done for almost every car, especially those working interchange service.
While it identified particular trucks for a specific car, it was an important ID for any off road maintenance or repair work, as well as identifying trucks of cars in wrecks, where they often are separated.
Further, the reporting mark and car number is also stenciled on both sides of the center sill for the same reasons.
While we deal with freight cars here, passenger and express equipment was similarly marked on the trucks and center sills.
Sometimes there was a plate on a side of the center sill that schematically outlined the brake rigging, if it had a unusual or complex arrangement.
Each journal bearing was also marked with a number on the truck frames.
They would be numbered 1-4 for the four wheel truck at the A end of a car, and 5-8 for the companion truck at the "B" end.
My recollection is a bit fuzzy here but I think the left was odd numbers and the right side was even numbers, front to rear.
The same was done with 6 wheel and larger trucks as well.