The 1st printing can be identified by the diagonal "rows" pattern found on the reverse side of the paper. It displays a lighter orange/gold than the reprints and measures 14" x 20".
The 2nd printing bears union logo 221 in the bottom margin and does not show the rows found on the back of the 1st printing. On this printing, the distance from the upper corner of the "T" in "Presents" to the bottom of the "y" in "Berkeley" is 19 5/16". This poster measures 13 15/16" x 20" and was printed after the concert, sometime between 5/6/1966 and 10/23/1966.
The 3rd printing was also printed after the concert, but bears no union logo. The identifying distance as described for the previous printing is also 19 5/16". The orange/gold background is slightly darker on this 3rd printing than on the others, and it measures 14" x 20".