David,
To the best of my knowledge, and my 55 was restored by the owner of a smallbird specialist restoration shop, the MC was left unpainted. I have seen guys who have primed the outside of the unit and spray painted it a casting grey. You can get a spray bomb for cast grey at any good auto supplier. The cap was not painted either, but is of an alloy or a grey pot metal of some kind and won't corrode. It almost has an appearance of being made of lead, which of course it isn't. If you have a serious show car, leave them unpainted.
Gene
