Bob,
The ejector (which is what you picture here--I make the same mis-type all the time) is a different quality and hardness of steel from the receiver extension. It is almost a spring-steel. Thus, it 'takes' salt-blue differently from the other parts of the gun, and by example is much more sensitive to the slight chemical and temperature differences which cause coloration ("plum", for example) other than the desired blue.
--Dwight
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