On the bottom of the intake would have been the Ford part number.  While the ’57 intakes are reasonably plentiful, a ’56 dual quad intake would be much harder to come across.  If the intake itself was in good condition, then it would have been worth the asking price regardless if it was a '56 or '57 model.  If the single Teapot that was available was an original dual quad carb, it would have been a bargain at $100 assuming it was all there and had never been on fire.