This might help....
http://www.gibson.com/downloads/blue...nAcoustics.pdf
It's a guide to Gibson acoustic products since the dawn of time. It might confirm for you the exact vintage/ make of your guitar.
BTW, I'd be a bit leary of that dealer's offer. He's a business, and he's in it to make money. If he's willing to give you ANY guitar in his shop, chances are that yours is worth far more than any in his shop.
Good luck !!