[Ron Wood]

I think Heinz may be on the right track. Although I do not think there were any salts involved, it looks like it might have been a type of acid rust blue solution that was contaminated with a copper compound (possibly copper sulfate that is a component of some "cold blue" solutions). It obviously was a wipe on application rather than dip since the color appears on part of the barrel and receiver, and the portion of the left side rail that is covered by the side plate is still blue. It is interesting that the frame latch for the toggle lock is also still blue and didn't "take" the red solution.

It is a sweetheart of a gun and I understand your affection for it. The fact that it has a Type I thumb safety with this late of a serial number is just another occurrence of DWM using up existing parts. I have a 1900 Commercial with a serial number over 500 lower and it has a Type II safety.