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Does anyone know the purpose of this flapper I am pointing at on the manifold for the post war straight eight engines and could this be the cause of some overheating on these engines?

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It is called the heat riser valve. If the valve is stuck closed it can result in overheating and also valve burning.

Purpose is to help warmup in cold climates. There is a thermostatic spring that when cold increases tension and moves the valve to the closed position. This forces some exhaust to pass thru an auxiliary passage in the manifold and up toward the carb to warm the base and surrounding area so the gasoline can vaporize a bit easier. As the engine warms, the thermostatic spring is also warmed and loses tension. That action lets the weight pull the valve open to close off the aux passage and allow the exhaust to go thru the normal route to the tailpipe.

In moderate or temperate climates some feel it is not that important. If you live in a temperate area and the spring is missing or the valve shaft is rusted and sticking you can make sure the valve is open and then wire it so it stays open. Once you are sure the valve stays open you can add to the list and repair as needed..

If you search on exhaust manifold heat riser valve, you'll probably find 100 posts about it. It's purpose is to improve engine performance at cold start up by using heat to improve fuel vaporization by deflecting hot exhaust towards the base of the carburetor. When it works properly, the thermostatic spring relaxes as the engine warms up and the counterweight drops, changing the position of the valve to "open" so the exhaust is blocked from the intake and passes directly into the exhaust header.

Sure, failure of the valve to perform correctly causes engine performance issues like vapor lock. Many owners, especially those in warmer climates, often either wire the valve permanently open, or even eliminate it. I use my cars in cool weather and a properly functioning valve does enhance performance during warmup, if it wasn't important to good performance, it wouldn't be there in the first place and I recommend repairing if necessary and keeping it functional.

PS - you're a fast man at the keyboard Howard, beat me to the draw again.

Many thanks to you both. My apologies for the redundancy of the post. However, you both helped me tremendously.

Sorry to hijack the post fellas but I came here to ask a question about this valve and lo and behold this thread was already here! So my question is, if you were to remove the thermostatic spring completely, would the valve then be in a position where the hot exhaust would be routed AWAY from carb at all times? Reason being, mine rusted and crumbled to dust when I touched it. I live in a warm climate so if this is the case I don't think I need to replace the spring.

You can remove the spring but it would be a good idea to wire the weight so it is hanging down to make sure the valve stays open. Without a spring vibration or a good jolt could cause the weight to bounce. If the shaft is rusty or starts wanting to stick a bounce could move the weight enough to start closing the valve and cause problems. A bit of wire prevents the worry. Max and Kanter have springs if you do want to get it functional.

Simply removing the spring doesn't assure you of anything, for example the valve could already be stuck or rusted in any position, open, closed, or in between.

Thanks for the responses guys. The valve seems to be completely unstuck and has a full range of motion. I can hear the valve flapper opening and closing inside.I believe i will wire it in place.