How was this part made

[Wongster]

Hello guys,

I just received some stuff from China which I ordered for rebuilding an old 3D Pinter. The 3D printer is a Mini Kossel using 20mm square aluminium extrusion as it's based.  The parts I shipped in include some aluminium corners for its base and top. These corners are to hold the alu extrusions together to form the frame of the printer. 

While inspecting the parts, I was wondering how they were made. The bottom corners are 60mm in height and the square hole for the alu extrusion has rather sharp corners. My longest mill wouldn't be able to mill so deep. I guess it's done in halves - flipping over the part after one side is done using the 2 round holes as index or a fixture with the profile milled in. .

But the square hole is a challenge to my inexperience mind. 

All the holes are clearance holes. Kind of a pain to set them up for drilling. 

Some pics:









Any thoughts?

Regards
Wongster