We had these custom hex shoulder standoffs manufactured in 316 stainless steel for a physical and accelerator lab in two thread sizes - M3 and M4 thread sizes with 6g thread fit.  The width across the flats on the hex measured 6.8-7MM and the main shoulder diameter measured 5.6MM +/-.05 and the step down shoulder diameter measuring 4.8MM +/- .05.  The internal female M2 X 0.4 thread was tapped to a depth of 4MM min.