Have not heard it in years but Bruyninckx says it was recorded in NYC on 12/26/68, 2/6/69 and 3/12/69. No additional vocalist is listed but I suggest it is Ivan Harold Browning who appeared on subsequent sessions.
"Is it worth it to check out garage sales and the like? Are there other online sources I should be keeping tabs on? Any help is much appreciated. "
You would need some weird-ass garages to find what you want.
1977, Blue Boye (Mbari, Screwgun)
2002*, The Collected Poem for Blind Lemon Jefferson (w/K. Curtis Lyle, Ikef) - Mbari, issued just before Dogon A.D. in '71 or '72. IIRC. It was sold out when I bought Dogon A.D. from Julius.