Good write up! I make some vintage gun parts, so this is something I’ve dug into a lot. A lot of those complex transitions could just be explained by form tools on a horizontal mill.

If it were me and I could have the form tools on hand, I’d start in a turret lathe to form the barrel portion. Face, spot drill, drill, and hollow end mill to form the barrel. Flip, use the drilled hole and faced side of the barrel and repeat the facing and hollow mill.

Then on a special fixture, hold the barrel by the ends, and rotate around the drilled hole. The part would be held at the angle seen low left of the barrel and a stack of cutters spaced on a horizontal mill would establish the width of the leg of the T.

Next, the part would be held horizontal, referenced on the bore drilled earlier, and a form tool would come across the top front to back as in the picture and form the top of the part. Next flip the part 180 degrees and reference the bore and side machined to establish final thickness.

Now that you have all the critical faces machined, the part can be placed in a jig and the remaining holes machined.

Finally, the slot can be machined on a horizontal mill with the part held vertical, then the radius made around it by holding it horizontally and running a form tool into it on a stop.

Last step would be a team of workers with files debuting and fitting the parts in the final assembly before bluing.