The normal stencil drawing for freight cars as built by P-S, SSC and others show that there were a multitude of stencils.
The truck bolsters (with car #)
The car ends (multiple lines)
the center sill
brake lever indicators
cast letters on truck side frames picked out with white
adjust brakes here
brake cylinder data
brake shaft end painted
On most of the stenciling drawings we have (and we have a lot scanned because, I always thought they'd be of interest so I pulled and scanned any I saw) they have all this and more. Then there are the more detailed drawings for the various plates, smaller stencils, etc. Plus the stencils inside the car that, depending on how it's staged, can be seen. The end of the car data (which started all this) is seen on every drawing.
Note though that these are AS BUILT. Fresh from the paint shop. What another shop might do afterwards, how grime and other weathering affect that is another story.
As Ted mentioned - you have to be able to match parts in case of accidents, wrecks, misadventures, shopping, etc. When an off-road shop had to replace trucks or wheels, the original casting #s had to be noted in the reports back to the owner, along with the new (if so ordered).
That's partially why the sill number & marks are there - why hatch covers get interior lettering, why interior doors get lettering, etc.