I just briefly posted some time back with an image of the Matrix poster that I bought myself. I have now had it framed and I have to say that it looks wonderful. The nice thing about having obscure taste is that some of the stuff that really interests me (e.g., New Riders) has less of a premium on it than more mainstream items (e.g., related to the Grateful Dead).
Sort of in conjunction, I picked up Grushkin's Art of Rock. "My" poster is in there at plate 2.118, but it is misidentified as 1969. I think this might account for the October 1969 listings that show up on lists based on Gary Jackson's Matrix Concerts data (e.g., at sixtiesposters.com). I had suspected that these were a duplication of the October '68 dates, and now am about 99% sure. Not sure the direction of causality (i.e., Grushkin -> Jackson or Jackson > Grushkin), but it's immaterial. As of now, I am no longer listing Garcia and Friends at the Matrix in October of 1969.