I worked for Masport Foundry (Now Precision Foundry ) for ten years as Senior technician and was involved with every stage of making these. They were originally shot blasted as opposed to sand blasted which will give a different surface finish. You are onto it with the washers however use stainless, as they will be much slower to burn out. With one each side and bolt them together with a stainless bolt, a little exhaust cement will not hurt either. Remove the wetback as it is made of copper and without water running through it will melt the first time you light it.
When you get a new damper for it get the hole welded up to about half it's original size, makes them a lot easier to shut down. Make sure you use a stainless rod for this job, but its not for the reasons given above.
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