318 passenger side manifold heat riser?

Does anyone know if all small-block manifolds came with a heat-riser valve from the factory?
Engine is from a 74 Valiant 4-door originally sold in Mississippi. The manifold appears to be cast to have a heat riser but there is no spring or counterweight and I can't see any evidence of a shaft on the front or rear. Looks like it was never machined. I'm second-guessing whether or not there was a butterfly in the manifold when I put the duals on and I'd rather not pull the manifold to find out.