Hose Bibb Mounting Plate

What is the pipe size, the big hole?

How does this protect dissimilar metals? copper pipe and alum. plate? i guess you need a gasket to protect the pipe from touching the plate.

Seems like there would be a market for decorative mounting plates. Anyone thought of this or seen decorative ones?

So is the plate made of steel or aluminum? Last question about this had two different answers.