[EC900]

7R is the case identifier…it’s the the strongest and last of the Magnesium cases, engine is 1977 model year. Your case could have at some point been long exposed to the elements. I’m not an engine rebuilder so I can’t comment on what may be the norm, but the casting could just be pitted by mfg. process too.
The amount of heat the engine creates would burn off any “preventative” external coating. As long as the case is true then the pitting is merely cosmetic. A mild, low pressure walnut shell media blasting would improve the looks, but mostly those pits will be hidden.