Frames cast solid and then the Machining begins. holes, along with decking the bottom and back, and also the Grip pin hole, trigger spring hole/ Cylinder latch plunger hole (NM).
Lot's of work here that can be done by a very steady hand, But best performed in jigs and w/ power machine tools.
Once one has the proper power tools and jigs it is quite a strait forward process.
How these get out is anybody's guess. I would guess, discarded for; warped casting, Porosity, and yes the good ol lunch box.
OK I went back and studied the pictures, look at the very front, left of the frame, Might deck out and might not. My guess not willing to take/ spend the time, effort on a maybe.
I have a little collection of these After Market Brass Grip Frames, this would be a new one as I have never seen a "JF" -vs- I have seen a few "SG".
I do have to agree it does look like a nice invesment casting, Now we just got to figure out who is doing em.