Interesting Comment, Tom. I found the halo ! !
On close inspection of the numbers on the gun, I find none of the "halo" effect on the serial numbers; but the unit markings have a clear "halo" effect around them, just as you describe it.
The only thing that bothers me is that the Unit Markings also have a good, consistent blue surface all the way to the very bottom of the impression, even the periods. I am hoping that this blue surface on the bottoms of the impressions is due to some kind of after-manufacturing treatment applied by the armorer who stamped the unit numbers/letters.
Forgot to mention that this gun had a Hold Open installed in some rework operation. This is clear, as it has the pin showing on the right side and a small acceptance stamp below it. This small acceptance stamp also appears to have been blued over. Curiously, the pin itself was not blued.
Luke
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