Sources:

1) Butch Trucks' comment on https://www.youtube.com/watch?v=ivDGQ21JgqI :
"That is not Chuck playing piano. That is Gregg. Sad thing about this rehearsal is this is where Duane took possession of the bike he was riding when he died. I remember how blown away he was when it showed up. This is also where we first worked on Blue Sky."

2) 'marionsindorf' posted on allmanbrothersband.com :

August 1, 2004, 07:55 AM:
"In Tom Nolans book Twiggs Lyndon talks about an early gig that the Allman Brothers played at a tiny junior college in Alabama. The students had voted to have The Lettermen appear at the one concert they held each year, but some hippies had stuffed the ballot box in favor of the Allman Brothers. Playing before a crowd who had never even heard of them the band saved Whipping Post for an encore. To their surprise the crowd wanted more so they went out and played Mountain Jam for a half an hour. To the bands astonishment the crowd whooped and hollered for more. So they went back out and played "My Favorite Things" by John Coltrane. Duane said "This'll cool'em off." And it did.
Now my question is this: I know that there are a couple of rough versions of the song on the Gatlinburg Tapes. And I've heard the song mentioned by Duane in an interview. But does anybody know if there are any tapes or bootlegs out there that feature this song."

August 23, 2004, 08:30 AM:
"Here is the actual text:
Twiggs remembers a junior college date in Decatur, Alabama, at a school with an enrollment of 500. The student body had overwhelmingly voted the Lettermen as the group it wanted to appear at the school's one official concert that year, but the two hippies on the entertainment committee stuffed the ballot box to make the ABB the winners. The group was hired to play before an audience who had never even heard of them.
"And the kids were really receptive, although I don't think they understood the music. The band saved Whipping Post for an encore, not expecting to get one. But they did, and Whipping Post got a standing ovation. They said, 'They really want us to go back? Well, let's do the Mountain Jam'. So they went out and played Mountain Jam -- for half an hour. Came back, said, they won't ask for more after THAT. But they did. So they went back and played My Favorite Things by John Coltrane. Duane says, 'This'll cool 'em off'. And it did."

3) The only known Allman Brothers Band concert in Decatur, AL took place on April 14, 1971:
https://allmanbrothersband.com/event/abb19710414/