Phosphate finishes are varying shades of gray to nearly black, and tend to become gray-green with age. If you've ever seen a WWII 1911A1 or M1 Carbine, those are phosphate finishes, also known by the trade name of 'Parkerize'. The picture you posted does not appear to be a phosphate finish on the upper receiver, although it does somewhat resemble a phosphated finish that is completely worn away. It actually looks more like a part that was left 'in the white' that has picked up a brown patina over the years. It's of course possible that the upper was phosphated, perhaps as an experiment on more 'military' finishes for the Luger. I know I have read here of phosphated Lugers, but they were all completely done.
If I had waited and read G.T.'s post, I would have shut up, as I think he has it nailed.
__________________
You can lead 'em to the water, but you can't make 'em drink.
|