A good gunsmith, proficient at gas or TIG welding can add material and dress it down to be virtually impossible to detect with the eye, naked or magnified.
However, a dilute solution of nitric acid, applied across the weld and base metal, will clearly show the weld, heat affected zone, and base metal, as distinct shades.

I guess it's an add on, it's a perfect job. I think what threw me off was the patina on the metal is not disturbed, so I'm guessing this was done many years ago and it all aged the same.
Since this is not original and I really don't care for it I will remove it. If this was an original oddball receiver that would be cool and it would stay, but it's apparently not so it's coming off.
Thanks, Tom

Tom, Little things like this are added character to the gun. Because you said the inlet into the original stock is a good job, I'd leave it alone. It will always be a conversation point. If it were an A grade, maybe even a B grade, it might be a different story, but D grades are relatively common.

I'd leave it too for it's uniqueness.

You would have to fill that vacancy in the stock and that would likely show to some degree.

Edgar
I follow your thinking, and I would but in the bigger picture I just bought this gun and when I got it home and examined it real well I found the stock is broken at the wrist. I took a beating on this purchase. My fault. I glued it up but am not satisfied with the result, so I'm thinking new stock and if I'm going there I might as well make it original.
Thanks, Tom