318 passenger side manifold heat riser?

1968-72 has heat riser valves for sure, don't know about other years. If there is no shaft, I can't see how the flapper would still be in. IMHO - GOOD RIDDANCE !!! I would bet what a former owner or mechanic did was do away with it and weld the holes shut, which is what I have done to every small block manifold that I've had. Pulling the manifold would not be needed to answer your question, just drop the head pipe enough to look or feel around inside.