"The History of Lite Beer" poster

My grandfather worked for an advertising company in Milwaukee, which was contracted by Miller Brewing Company. My grandfather is the artist of this advertisement (Kenneth Frank Sr.). I found the poster in my grandmas basement about 10 years ago. I thought it was cool, so 15 year lock me thumb tacked it on the wall in my room. About 3 years ago I realized that this is a potentially valuable piece, and had it framed and matted. I tried to do some research on this particular advertisement, but came up empty. My father remembers this particular poster from around the early 1980's. Here's what intrigues me about this. My dad and I think that this is the ONLY one of this particular poster remaining in existence. My grandpa kept it because he was a pack rat, and when he died my grandma just never threw it away. I'm guessing that whatever gas stations or liquor stores had this hanging on their walls, just threw it away when the next advertisement come in. That's why I think that this is he only one left in its existence. It's a neat poster, however, thanks to 15 year old me, there is thumb tack holes in each corner. Those are covered by the matting though. There is also a slight rip in the poster itself at the top, through the "I" in "history". This occurred before the poster was laminated though. The poster is approximately 31"x21". Framed it is 38"x 28.5"