Quamen have been making flatland parts inhouse in California all these years. Their set-up is comparable with the one of S&M, FBM, or any smelly garage company that wants to make things at their own place using whatever equipment they have available to make it work. Welders, benders, cutters, CNC machine, jigs, and metal pipes everywhere. You get the idea.
Here are two flatland products they've got in their line right now; the Clad G7 frame and the Mobile Armz crank arms.