Hi Greg-
Is the shot you are mentioning the one with the rainbow in the background? That shot is from August of 1971 according to the slide mount (unless the film sat in the can for years). As you say, perhaps the 497 was pulled out to make space in the roundhouse?
Regarding the RGS shots, all of those 40's and 50's shots were taken by other photographers. I have been fortunate to purchase shots individually and in collections and then take the time to scan and optimize each shot prior to posting them online (as I do with my own shots). I think getting these photos out in the public can only help keep interest in the narrow gauge railroads alive, hopefully for generations going forward.
Steve