This is not a gig poster in the true sense. This would actually be considered FAN ART. It was not used for promotion, nor was it sanctioned by the band. That is probably why it does not actually have the word PHISH on it. .. 'cause then it would be (more blatantly) infringing.
I'm a little ashamed to say that, although it took me a few seconds, I did get the reference... I went to one phish show with my little brother years ago, and I remember the drummer wearing this dress. Not a fan, but I think it's a great poster... and it's kinda cool that many of the phish posters don't need to say "phish" on them, and fans just know... it's like hobo language or something.