I assume y'all know Merl's Tune, although there are unfortunately only a small handful of known performances:
3/7/73 - with George Tickner on 2nd guitar.
4/14/73 - no circulating tape, also with Tickner
7/5/73 - released on GarciaLive vol 6
7/10 & 11/73, i.e. the various Live at Keystone releases - Merl's Tune wasn't on the original album and didn't see commercial release until the first Keystone Encores LP/CD in 1988.
11/3/73 - I believe this was Bill Vitt's last show (or one of his last) with Garcia/Saunders?
I am a big fan, in particular, of 3/7/73 and 11/3/73, but of course all of these shows are worth hearing. For some reason, Merl's Tune fell out of favor (or maybe was never in favor to begin with?), but it would have fit very well next to the band's jazzier material in 1974-75.
Check this out, though. Cue up any of those version of Merl's Tune, then take a listen @20:40 here:
"Help Us Out," credited to John White (per label scans at discogs) vs. "Merl's Tune" credited on Keystone Encores to Saunders/White. Thanks to Youtube commentor michael tristan for pointing that out (um, 6 years ago), and to Peter Tschirky for posting it.
John White is a little-known Bay Area guitarist with, afaik, this one Mainstream LP from 1971 to his credit. I don't know anything about him besides what's in this older blog post: http://soundological.blogspot.com/2009/09/john-white-john-white.html
Saunders plays on the whole record, along with drummer Philip Wilson (ex Butterfield Blues Band, and later a renowned drummer in free jazz circles) and saxophonist Hadley Caliman. Mainstream's 70's jazz catalog mostly has not made the transition to digital yet, and the original LP's were usually pretty light on detailed session info, so no luck there. The dense horn-heavy sound on this track is also typical of Mainstream's overstuffed approach to production, but imho this record definitely fits well in that gritty Bay Area rock/R&B/jazz guitar bag next to, say, Harvey Mandel or Mel Brown, that kind of thing.
I have heard a couple of Merl Saunders sideman appearances from this same Garcia/Saunders timeframe, but the ones I'm thinking of were for Fantasy Records, Saunders' label. This John White LP is from 1971, so presumably Saunders wasn't under contract with Fantasy yet.
So, um, yeah. Dunno if this is in the same league as the "Finders Keepers" misattribution. Barring other information, I will just offer this for your consideration.
update (Dec 2019):
JGMF sent along a pic the tape box for one of the Bettyboard reels from 7/11/73, with this handwritten notation of the contents:
Harder They Come
John White Song
Like a Road
How Sweet it Is
My Funny Valentine
Okay. John White song. I guess that answers that question? Maybe not?