The photos in the
The Mudhens from the Summer of 1941 of 459 in Flying Rio Grande livery would seem to show the right side panel as a steel sheet and the left a recessed panel in wood. Further evidence of this would be the smaller "459" on the steel side, matching the numbers on the paneled side. The other steel paneled K-27, 461, got the big numbers when the new scheme was applied.
An assumption but it would seem to be a logical answer.