As with Dick Harley, late in commenting. I took a broad look over some photos I have collected over time. And uploaded some below.
Of these photos there is consistency (with the NP shop procedures) that not only the ends had data painted however even the truck bolsters can be seen in some of the photos having the car number painted on. To those who might talk of wheel bearings throwing oil out and obscuring the stenciling, the lettering is on the inside of the wheels where most of the spray might not land.
Will I be lettering the truck bolsters on all my NP cars? Decals in the correct sizing would be a challenge to find, so perhaps I will do a few resin kits.
On the prototype which should be the basis to study, there it was.
NP 24798 - note on the truck bolster car number is clearly visible on this 1947 Laurel, MT kit-built car. (The shops assembled parts like doors and ends from builders, however crafted the sides and did the assembly.)
NP Covered hopper 75040, built 1951 at the NP Brainerd, MN shops.
NP 84000 - yes the roof, ends, and even the coupler box is covered with aluminum paint, which makes the end data pretty visible. Cars were listed in the Mech. spec sheets as having aluminum paint to protect pigs from excessive heat. (Sides, trucks, underframe - BCR.) In addition they had counter-balanced shutters that could be moved to enclose the car entirely in the winter climate local to the NP. These cars were original NP 8000 series XMs of 1929 and rebuilt into this car in the photo in 1958. Roller bearings and snubbers on the trucks for a smooth ride.
3309 Built 1-1960, so it barely fits in here with respect to this lists time frame. End data is there.
I have other photos from well beyond this lists time frame that shows on the NP, this practice continued.
Last image - I am not sure if I posted this image prior. It is a response to a question from Lester Breuer that I may or may not have answered. The time frame covered by the letter affects the dates of some who model this and earlier eras here and so I will include it.
To sum up, the words of Garth Groff on check your photos seems to be the way to go. Jim Dick - Roseville, MN