I see you have studs holding the head on, not bolts. Are they 7/16 inch or 1/2 inch? If they are 1/2 inch you may have something interesting there. I think 55-56 engines came with 1/2 inch studs from the factory. Your Twin-H manifold is from one of the earlier cars but that doesn't mean much. You could also have an engine somebody modified or even a 7x but chances of a 7x are very remote. Even if they are 7/16 it means somebody was in there and replaced the bolts with studs. I am very curious to know what you have.