I could have posted this over in the Leica M forum, but I figure if anyone knows anything about black paint M2's, it's Tom.
I've seen both (claimed) original black paint M2's and (claimed) Shintaro black M2 repaints in which all the parts appear to be black paint over brass except the retaining screws (I think that's what you call them - maybe it's retaining rings) for the self timer and preview levers on the face of the camera, which appear to be black paint over chrome, or at least over a silver metal.
Here's an example: black paint M2 on ebay
and here's a Shintaro repaint:
And it's not just the way the light is reflected. I've seen the Shintaro repaint in question and it's definitely silver metal there despite brassing elsewhere, and the one on ebay looks the same to me.
My question is, were these parts not brass? I know Shintaro dechromes all the little bits and pieces before painting, so either they're not made of brass or they don't get dechromed for some reason, which seems less likely.