This is the best place I could think to post this, but if there is a better location, please let me know.

A while back, I got one of the unissued CZ-52's that came on the market.  It came with perfect gray parkerizing.  Anyway, I stored it in a plastic foam-lined pistol case.  I know this isn't the best idea, but it's the best I can do right now, and I always kept the gun well-saturated with CLP.

I recently checked on it, and discovered that there appeared to be a pair of large brown rings on the cocking serrations, where one might place their fingers while cocking the gun.  My first thought was rust, but I discovered that it was in reality some kind of sludge that could be wiped off with a paper towel, or scraped off with a toothpick.

I got the sludge removed, but discovered that the area it had surrounded had lightened significantly in color.  (I also noticed that the part of the case foam that had been in contact with this area had a brown discoloration, as well.)  I applied more CLP to the area and let it soak for a few days, but it didn't serve to darken the color any further.

The area doesn't appear to be bare metal.  There is also no sign of rust or pitting in the area.  Even if nobody has an idea as to how I might darken the area again, I'm interested to hear any ideas as to what might have caused this in the first place.  The only cleaning/lubrication products I use are Breakfree CLP, Sharp Shoot R foaming bore cleaner, and TW25B, and I've never had a problem with them in the past.