Jason,
461 was leased to the RGS when that picture was taken. I haven't had the chance to look at any of the other leased engines on the Southern to see if they have the same mark.
Could it be a mark that the D&RG painted to the cab to denote it as a "Loaner"? I will look around to see what I can find and I'll let you know what I find out.
Have fun in Chama.
Duane