I hate to tell you Mike, but your cars have used foreign screws for the last 30 years. Not that American cars are made in America these days, but still. Wouldn't surprise me if there are more inch dimension threads made overseas than in the US at this point. Having said that, screw production is actually still alive, well, and cost competitive in the US. Probably in part because it's so highly automated at this point.
What's really frustrating is when you call Ecotech directly, ask them what thread the main mounting screws are and they tell you #10-32. Then you call back a little bit later and they say M5 x 0.8. It's M5 for anyone curious.