I think they also were prone to rusting and tossed when the mounting ears gave way.Yes, indeed. Without that heat shield the coupler will soon be baked. My 70 has the cylinder style that you mentioned. Removal and installation of the thing is a bit of a fiddly pain, and I imagine many were tossed aside by a mechanic working on "book time" many years ago.
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It was missing on my car, most likely left off or removed during repair. I'd wager that some bought the farm during engine swaps too as they just look like something the engine would hit while swinging from the chain.
Bottom line is they are a bit fragile. I'll even say that the mounting ears were damaged by overtightening to the point of not being serviceable after a coupling replacement when it was installed on the assembly line. Of course, the cars weren't meant to last 50 years either and many cars never had the coupler changed in their lifetime.
I had a hard time finding a decent replacement and even that needed some work. This would be really good part to repop!