Based on the cleanliness of the entire engine consist, I would say this had to be the return trip of their maiden voyage to Daytons Bluff. They never looked so clean afterwards. 949 & 951 both have a builder's date of May, 1962 so I would go with that date. The story goes that engineer Henry J. Groesch ("Izzy" aka, "Isadore") was given a message (signed by Aurora RFE McPherson) at Savanna Yard, informing engineer Groesch that the PR Department photographer(s) would be in the vicinity of Naperville to shoot publicity photos of his brand new locomotives and that engineer Groesch was to keep his blankety-blank "mug" inside the cab while passing their location. You see the end result. The RFE forgot to mention, "without fail" in the message. Volumes could be written about "Izzy" and his son, Lawrence L. Groesch (no Jr.) aka "Lil' Izzy". Legends.
RC