I don't know who the Leica specialists are, so I suggest asking here on Apug and some other forums, like RFF. Also, there might be someone near you who is a specialist and could help you.
On the Fed, it could be that it is only a chrome layer, with no nickel. It's also most likely that if a nickel layer was there and the plating peeled off, the nickel layer and chrome layer would have come off together. Chrome layers are thin and surprisingly porous, and don't usually separate from the interlayer. When chrome plating peels, it usually comes off the base metal, though I have seen the nickel layer separate from the copper layer, leaving the copper layer intact. That was on steel, which had been distorted by impact.
Most plating layers fail from underneath because of poor adhesion or corrosion under the plating, or both. Layers can also fail from excessive flexing or distortion of the part. I think the loss of plating at the portholes is probably because of corrosion starting at the edge of the holes, where it can get under the plating.