The lettering is the latest air brake test... As others have noted it would be too small to be legible on a model, but for preservation its a wealth of information about the car's regular inspections, so we sand carefully down through the layers recording the information found, typically a date and location, with the lack of a current date showing any periods when stored, so no air brake test, or when the last test was preformed.
From the late steam era, it was called a COTS for "Clean, Oil, Test, Stencil" (with a date) Currently, (beyond the dates that this group covers) it has become a "Single Car Air Test" and is simply stenciled "ABT" and a date. Current rules call for a test based on miles, with a test every two years if the car doesn't get used much.
