I called it tin as I couldn't find the proper words to describe the finish. It could well be a zinc or tin coated steel and it weighs a lot less than the original. I was surprised to find the metal surface shiny and corrosion free which is a big difference from my original which looks to be steel with the bottom rusted out. The corrosion has come from the inside, which means that it picked up a lot of moisture from somewhere.
Thanks Jeff for the info on the white mark and yes, I have the correct mounting parts.