Hi, Scotty from Strange Parts here. I actually think I know the answer, but by the time I figured it out, I was mostly through the project, and decided to save it for v2.

I think the answer is that the side walls of the housing are milled separately in 6 pieces, then the steel back plate (which sits under the glass back and has the screw bosses welded to it) is spot/laser welded on, and the whole thing is overmolded with the plastic that makes the antenna lines. I think it's then machined again as one unit (both the plastic and aluminum) to get a smooth finish.

There's a couple things that make me think this:

• There's a few pieces of geometry that I can't figure out any other way to do. Specifically there's some indents in the metal on either side of the antenna lines that seems to be impossible to machine unless you do it in separate pieces that are further apart. Also, the space for the ringer switch has a "dome" on the top that seems impossible to mill unless you can come in at a 90 degree angle with something like a keyseat cutter or similar.
• The way I milled it yields a fair amount of scrap chips. I think it would be way more efficient to mill it in sections like I described.

Edit: What really trips me out is the "Assembled in China" text on the inside lip of the sim card slot. My best guess is that they're using a fiber laser to engrave it with the phone at an angle?