No idea if it is original but two interesting things.
1. An in store promo poster that was never displayed and in that shape from over 50 years is pretty rare.
2. Why would anybody do a reprint of a poster that has a fan base on fixed income!
The poster is also on ebay. The seller has several other A&M promo posters for sale along with other (mostly) legit looking memorabilia. Hard telling how peole come into this stuff.