a bad batch of stainless Ed Brown recieved ~3 years ago (from Ruger I believe)...greater than average carbon content apparently - although Ed never elaborates too much on things.
I own many EB guns and had this happened to the beavertail and MainSpingHousing on a brand new Stainless Kobra Carry within a few weeks of carry/use back in 2005. I spoke with Ed Brown personally and he had the pistol shipped to him and back in 2 days in better than new condition.
Apparently, it can be fixed with vinigar and a stiff brush, but I didn't think it was up to me to make it right. I am sure Marc M. at Gemini will take care of you...from what I hear he is a top class person/smith! Ed will replace the part FREE of charge, but it would require Marc to re-fit it.
From everything I've seen and buying 3 EB's since then (2 stainless), I have never seen this crop up again. I suspect it was some older inventory at Brownells or Midway that Marc happeed onto...
Here is a pic of my issue, fyi:
