Austenitic stainless steel (the type most likely used for clip screws) is also non-magnetic, so this test will unfortunately not narrow things down much.
That thread reports that the McGizmo screws are titanium but the screws sold by the Sandwich Shoppe when you buy a titanium McGizmo clip are stainless. If memory serves me the Arc6 screws have a flatter head than the McGizmo screws and would seem to be consistent with the ones sold by the Shoppe. To sum it up, I think they're stainless.
As an Amazon Associate we earn from qualifying purchases. Product prices and availability are accurate as of the date/time indicated and are subject to change.