Sunday River Productions was Alva Morrison, who shot his own footage on 16mm, then would record with a small recorder and splice it in. For other stuff he acquired like the vintage steam-era material I think he just put in whatever he could. He passed away a number of years ago, but I think his sons, or other relatives have kept it going but suggest you google to get more updated info. My brother ran into him in the early 70's in South Africa shooting steam there. I once rode a steam special from Cumberland, MD to Pittsburgh and Alva was chasing on Sand Patch with a Helicopter. Really ruined my friends audio recording on that steep grade. I had silent movies(Last year before sound) so mainly took slides from the window. I last talked to Alva in 1985 while we both shot C&O 614 on those coal trains during January of 1985. Now there was a spectacle to see in the winter!!!
Greg