I just stumbled across the coolest album that I can't seem to find anywhere besides youtube.
From what I can put together from the notes, this would have been recorded in the early 60's. Christopher Lauba is listed as a performer, and he was principal horn in between Farkas and Clevenger, so as well as I can make out, early 60's.
The piece in this "video" is Alexander Mitushin's Concertino for Four Horns. Enjoy the unified sound and some really fantastic low horn playing!