Anyway, TJS has listed a Garcia/Wales show for Sunday, June 7, 1970, probably based on a listing in the Berkeley Barb contributed by Joey Newlander. Gordon Sharpless was also credited as an information contributor, though I don't know in what capacity. Here was the note for the show:
Billed as "Jerry Garcia, Howard Wales and Friends". Listing from newspaper advertisement. The Dead played this same date at the Fillmore West. Although it's a possibility both shows took place (this being in the afternoon), it's unlikely.
As Ross points out to me, Sunday shows almost never happen at the Matrix. Gary Jackson's list of Matrix dates has the club closed on June 7th. The GD were playing at Fillmore West, as noted above. Finally, there is good evidence, apparently including a recording in the Garcia vault, for a June 8, 1970 Garcia/Wales gig at Matrix, their usual Monday Night thing.
In brief: seems like there was no HWJG thing at the Matrix on June 7, 1970. I'd guess that the Barb had a typo or other misinformation.