Round number plates

Keith,
You need to take a closer look at the locomotive's number plate. She has a rounded plate. 318 has the rectangular plate.
The story as I have been told was that during the era these locomotives were built (1870s and 1880s,) they were assigned to a specific crew. Since the crew had their own locomotive, they tended the treat it like a pet and purchase special details like better paint, fancier detailing, better whistles or ROUNDED NUMBER PLATES. Whatever the D&RG allowed. These men literally lived their lives on these locomotives as there was no such thing as the "Hog Law."
The engine when manufactured by Baldwin could have been shipped to Colorado without its number plate.
When she was placed in service, from the photographs I have seen, she had a round number plate with raised letters of 409. When the locomotive was renumbered, they chiseled off the raised numbers but retained the round plate with raised brass rim. From this day on, the number has been painted on her number plate.
The reason not all locomotives had these fancier number plates is because not all crews were so good to their locomotives (or wanted to spend money on their machines) back in the early days of the Denver and Rio Grande.
Check out the book about the 346. I believe it is sold in the museum and I am pretty sure there are copies in the roundhouse.
This, of course, is my understanding.