Sorry guys, but you are both wrong!!. I have taken some shots of the manifold too show what I am talking about.
As I said, the water jacket runs along the outside of the manifold from front to back.
The first shot shows a small pile of rust that came out of the jacket after I pocked a rod through.
By the sound of it, this may be something out of the ordinary, as this is the first time I have seen it on any grey manifold. It would not have been for the linkage rod, as it is to far away from the carby bases to be connected.
Regards, Jim