The piece looks Brazed/Silver Brazed/Silver Soldered (Hard Soldered) into place. Whatever term you may want to use, it was/is a high temp process. Depending on which of these and the specific alloy the flow temps can be around 1100 to 1600F.
Which ever,,The extension would be slightly oversize in my way of thinking. Perfect in how it fits to the frame contour, to get nothing but that hair line of braze to show betw the two parts. But slightly oversize to the grip flats.
This is where the oversize matr'l would be machined down to meet the orig grip flat surface.
Start with a Mill perhaps or go right to a surface grinder. The marks seem to indicate the latter was at least used to do the finish up work.
In doing this perhaps the orig grip flat surface was taken down a couple .000 or so to make sure every thing was nice and even.
Not enough to make a big difference when measuring the tang width now, but just enough to make trimming the side(s) of the springs a couple .000 as well to make sure they didn't touch the back side of the grip panels.
The jeweling a quick added attraction after polishing the springs.
On those surfaces it can be done hand held under the spinning jeweling head. A couple minutes work at best.
The Frame would have had to be polished at least around the tangs to blend the new extension metal polish with the orig tang polish. The gloss and grit marks have to match.
The rest of the frame if not heat damaged from the Brazing would be just stripped of the blue and the surface brightened back up. The new work area and orig polish brought to match once again
With my little experience with Charcoal Blue and Carbona, neither takes well over 'old bluing'. In other words it doesn't touch up easily as Rust Blue can or even Hot Salt.
So I would say either the entire Frame was stripped and repolish and the frame w/ new extension re-blued.
Or....
The extension was added to the Frame while still in production,,still 'In the White'.
That way the entire Frame got it's final polish and then factory blue (Charcoal or Carbona) and that is what you see..
Not added to an already finished and blued gun, but added to an In the White gun during production and then final finished.
The 'Why would they do this' and When,,I have no idea. I'm not the collector.
But that's what I see and I think that is what went on here.
