I also have this 1975 Columbia double-LP stashed somewhere around the house. Imagining this was the main representation of the Thornhill oeuvre that was to be found in the 1970s bins, or were there any other compilations that circulated?  

Joseph Orange:  https://research.library.fordham.edu/cgi/viewcontent.cgi?article=1186&context=baahp_oralhist

Listening to a Claude Thornhill anthology that provides a nice overview of his prime years, from the mid-1930s into the early 1950s. Caveat that it's a 2015 Acrobat CD-R release, but it was seven bucks new at my local record store: Praise always to the late Alastair Robertson for having shepherded eight CDs of Thornhill material into being through his invaluable Hep series, though I still wish Mosaic had been able to usher something of its own into existence--but I imagine the market for a Thornhill set might have been thin even 20-30 years ago. The individual Hep volumes remain my go-to Thornhill CDs, but this is perfect for an anthology mood (and about half of the first disc consists of sideman and early leader dates that took place before 1940, when the Hep series begins chronologically with the Snowfall CD).

https://en.wikipedia.org/wiki/Automatic_double_tracking The technique was first used by The Beatles on Revolver. 1966 was a few years after any of the three records in question, so homemade does seem likely. Is there any further provenance on the tapes? And what brand is the hardware? Sony used to make RTR decks that would allow "multictrack" recordings on two-track tape by bouncing one track over to another while recording a new one onto the track that was being bounced. I don't think that this is exactly the same technique though. And 15 IPS on the home market was pretty rare until somewhere in the 1970s. So, I was guess homemade, but.. in whose home?

I always wondered why there were two covers for this album. I'm sure the story is out there. It was beautifully recorded. I spun it twice today. Shepp is great, but Curson...is not having the best day. His chops are spent at the end of Girl From Ipanema. And what happened to Joseph Orange?