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?