Help me on a piece of music's art history....

I recently and for reasons not known to me stopped into a small antique store while on vacation.....I saw a poster that caught my eye. It was the billboard poster for the 1972 Watkins Glenn '' Summer Jam'' concert that featured three of my all time favorite artists ; The Grateful Dead, The Allman Brothers Band and The Band....it ended up being larger in population than Woodstock. It is in great to mint shape and I am 98% sure this is not just a recent reproduction but very well could be the first run of this poster. I went on the Internet and saw where the promotor of these bands the late Billy Graham, ( I do believe the site was called Wolfgang Productions ) had one which is probably the original and it was NOT FOR SALE !!! He had other concert posters from ''that era '' for sale on that site. This could be and I am hoping that this is the first run of these posters used for the promation of that concert. I was almost 16 and had my backs packed to go along with my brother and his friends and my mom stopped that from ever happening...NO it is not for sale but does anybody have an idea of it's worth or who would possibly know if this is in fact the real deal....!!!!