Heat shields are usually made from a light weight, reflective material and are installed to allow air flow around the item being shielded. Since these are thick metal, painted black, and are a tight fit over the mount which prevents air flow and would trap heat I very much doubt they have anything to do with heat protection.
I suspect they are the first design iteration of what is now known as an engine mount support cup, which became standard on mk3's and are available from Burtons and others as after market spares. The current ones are full depth but ours are only half depth so they would have been installed to give the mount support where its needed, which is where the lever arm is the greatest, which is at the cross member end facing up.
Since they are a snug fit, coated and installed at an angle the chance of water ingress and corrosion would be minimal. The weight of the engine on the mount is causing a tight seal at the bottom. After 10 years as a daily driver mine didn't have any rust.
If ours were to be installed at the engine end facing down, it would effectively stiffen the top half of the mount making the sheering at the cross member end worse, and if extractor heat was a problem, which based on the distance apart I doubt, they would trap heat and cook the top of the mount.