The (admittedly poor quality) clip above is from Ralph J. Gleason's Ad Lib section in his San Francisco Chronicle column for Wednesday, August 6. As always, he lists rock and jazz highlights around the Bay Area for the next two nights (his Friday column discussed the weekend). It says
at The Matrix tonight and tomorrow night New Riders of the Purple Sage (W. Jerry Garcia). The Matrix had its sound equipment and tapes stolen last weekend, incidentallyThis listing confirms that The New Riders of The Purple Sage played The Matrix both Wednesday and Thursday nights, August 6 and 7, and in my mind locks in the accuracy of the August 7 New Riders tape.
More interestingly, I am confident that this is the first appearance of the name "New Riders Of The Purple Sage" in the press, and probably anywhere. At their previous gig, on August 1, they were billed as Marmaduke, Jerry Garcia and Friends. In the intervening time, Robert Hunter seems to have made up their new name, and they are promoted as such. More distressingly, it suggests that some tapes were stolen from the Matrix.
It is slightly contradictory that the New Riders opened on Wednesday August 6 and are listed on Friday August 8 as opening tonight. However, the Friday listings were probably typeset sometime in advance, and its possible that the Wednesday and Thursday shows mentioned by Gleason were added later, as he had a shorter time frame. Knowing, however, that neither Garcia nor the Dead had a known gig on the weekend of August 8 and 9 makes me believe that the New Riders played from Wednesday to Saturday. For the record, Gleason doesn't mention a Matrix performer in his Friday column, but its hard to draw a conclusion one way or another based on that alone.
The Matrix generally had the same performer on Friday and Saturday, so if you accept my hypothesis, than the Saturday night (Aug 9) seems as likely as the Friday. Besides breaking in the new band, Garcia was probably looking to do the Matrix a favor if they had just a bunch of sound equipment stolen. If anyone has another theory about how the weekend shows may have played out, please suggest them in the Comments, but for now I am going with the New Riders for 4 nights at The Matrix from August 6 through 9.
The billing on Friday says The New Riders of The Purple Sage with Jerry Garcia and Mickey Hart, but not Phil Lesh. This leads me to think that Mickey Hart considered himself a member of the band, just like Garcia, but that Phil Lesh was just a temp. Both Bob Matthews and Robert Hunter rehearsed with the group, though they didn't perform much or at all, and Dave Torbert joined in Spring 1970, so perhaps Lesh's status was understood from the beginning. Hart played drums for the Riders until late in 1970 (I believe Hart's last show was November 29, 1970 in Cleveland).
Sunny Land is probably an act called Sunnyland Special, who opened a number of rock club gigs around this time. Whether its the name of a group or singer isn't clear to me.