Not what it seems, What is it?

I have a poster which I had always assumed was a promotional 18" x 22" reproduction of the actual Rolling Stone Magazine issue cover Dec. 1st, 1977 #253. Recently I discovered this is not the case. The artwork is all the same as the published cover, but the text is different on mine. There are also some other oddities. Even though the date at the top of the poster is December, advertising in the lower right corner is for November. It was framed by Rudy Blum, Art Concepts on Sunset Blvd., LA. I am acquainted with Mr. McEuen's brother and he thinks it may be a proof of advertising for the 10th anniversary event. I have been unable to find any reference as to what it might be or anyone with a valid opinion. It certainly seems to be unique. could it be one-of-a kind? So far, Rolling Stone has not been willing responded to my requests for information. I have not tried to contact Mr. McEuen for an explanation. I hope you will not be too busy to respond if you know anything about this item. I will certainly value your opinion.

I received the poster along with a few other items as gifts from William E. McEuen, who was Steve Martin's manger at the time from a storage unit he had in Aspen, CO. That was in about 1977-1978.

Please excuse the photo quality, it is in a plastic frame and I don't want to risk damage by removing it.

That is about all I can tell you about it.
Thank you for your time and have a good day.