After a bit of research I think I've nailed it down to October 31 1968.
Jeff Beck played a benefit gig for the children in Biafra at the Fillmore East on the 31 October 1968. I believe PG was managing the JBG at the time? Regardless, Jimmy and Jeff were old mates who probably wanted to catch up (I wonder if it was this occasion that Beck first heard LZ1 and their version of 'You Shook Me'??)
And the 'mystery man' in the middle I believe is Steve Weiss, Led Zeppelin's US attorney.
Also if you look at the official timeline, there is a gap inbetween gigs from October 25 to November 9, 1968, hence Jimmy's presence.