Hi, Greg,
My first response would be that it is probably a train running after Labor Day weekend, when the gons might have been thought to be too cold. I remember very well that in the later 60's, gons ran in all trains, I don't think I can recall a train that had no gons at all.
After Labor Day weekend, the D&RGW ran only one train on weekdays, 2 on weekends.
With only three gons prior to 1967, one of the trainsets would have had only one gon, and it's possible there might have been a mechanical problem which forced the removal of that car.
Look for fall colors and at the windows in the coaches, if they are down, it's a pretty chilly day, especially if it's in the afternoon. These things might help pinpoint the time of year more closely.