The initials are likely A.M.C. of the person. Last name beginning w/'C'
A Monogram of those would make them ACM.
The last name initial is placed in the center of the 3
and is made larger in size than the first and middle initials on either side of it.
Jewelery Art & Lettering stuff.
And having said that, and also having never even thought about it before, it occurs to me our rather disrespectful term for an engraved gun ("all scratched up") may very well have originated via "through the blue" work.
One of our more knowledgeable folks once noted bluing is about .0002" thick---tough stuff, but not much of it. Given that, it certainly wouldn't take a whole lot of time/effort with the appropriate tool to scratch through the blue. Talent and skill, yes; but not a whole lot of time/effort. Perhaps one of our more knowledgeable engraved gun fans can confirm that.
And I thought I had a great BBQ rig with my Engraved 150th anniversary Texas & Southwestern Cattle Raisers Kimber 1911 and hand tooled holster from Georgia.