I think it is largely a problem of ignorance of the lettering practices of the '30s. Very few pictures were ever taken of the rear end of tenders. Sometimes we get lucky when an engine is backed for a while, like in the stock shot that Jerry so graciously referenced but for the most part the photos we have to work with are front end 3/4's. Most modelers didn't know about the little numbers and had only seen the Flying Rio Grande scheme in person.
When I did this one in Sn3 in the '70s I had never seen the rear of a '30s tender so, of course, it has a big "223" on the rear end. Some day maybe I'll repaint it.