Earl,
Actually, I think the larger numbers on the tender rear came earlier with the change to the Flying Rio Grande scheme. You can see photos (approximately 1940-41) of locomotives side by side where the new Flying Rio Grande locomotives have the large numbers while beside them the old herald scheme locomotives do not. A photo of helpers lined up in Sargent comes to mind. Also, there are photos of locomotives on the Santa Fe branch with the flying RG and the tender numbers-- obviously pre 1941. Before the flying RG came in with the larger tender numbers, there were smaller numbers on the tenders with the herald schemes; usually centered at the bottom of the tender. Again, you can see these smaller numbers in photos. Hope this helps.
Bill M.