I'm usually better at accepting a not exact finish on the hardware than I am at a difference in fastener design. I know the bean counters at GM would not have used a wide flange screw unless they thought it was necessary. Lots of parts in these hideaway systems that are grabbing thin'ish stamped sheet metal parts that need all the support they can get.
I bet these adjuster nuts are kind of an industry standard hole size ... so possible numerous different types and suppliers.
Jeez, looks like these guys have it covered
https://cliphouse.com/HeadlampAdjust...ts.aspx?page=1
Now that I look, I see tons of them everywhere ... gotta love the internet.