There are no copyright marks on the poster. It can be dated by the performance days, shown and by this link to a now defunct Fillmore website which shows a smaller image of the poster and gives the dates as February 4, 5, and 6, 1966. This was the first non-benefit concert held by Graham at the Fillmore. Graham moved his concerts from the Fillmore Auditorium in July 1968. After the move, the venue was renamed Fillmore West, as he also opened in New York City by then at the Fillmore East.
A copyright search on Bill Graham turns up sound and motion picture recordings and concert posters with much later dates, but nothing pertaining to this particular concert poster. There is no evidence copyright is or has been claimed on this image.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.