Probably when the first 2-8-0s were built.
However, if you're referring to Consolidated Stencils, they became requirements in their white-on-black form during the mid-1970s, possibly around 1974 or so. This was originally information that was haphazardly stencilled on the bottom of the car somewhere (except for the build date, which was added to the stencil later).
One company that had the equivalent block of information on its cars much earlier was Pacific Fruit Express. Perhaps one of the owning railroads of the time (UP or SP) also did something similar.
Carl
Railroader Emeritus (practiced railroading for 46 years--and in 2010 I finally got it right!)
CAACSCOCOM--I don't want to behave improperly, so I just won't behave at all. (SM)
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