1918 DWM, Reblued?

[barr44]

Alvin,
I have a 1918 Erfurt, which also has some color variation with the frame bluing. The inside of the side-plate is darker than the outside. I would suspect that being more exposed to the light and to normal wear and tear, would have some effect on the bluing. Comparing the internal bluing on the side-plate to the frame bluing indicates that originally the bluing differences would be less apparent. I suspect that something to do with differences in the metal or the hardness would cause some of this fading effect, but again I'm not a metallurgist.
For what it's worth, barr