Edit : after our chat on MP, that screw remains a partial mystery. Ive mistaken the place you were talking about. The standoff,as you said is based where you added that tiny yellow circle that I just find out.
Anyway, that screw is still in my opinion the same type of screw than before but maybe an M3 since this size of that standoff should be an M3 threaded.
Regarding the lenght, it's just a matter of measurement. Buy 12, 14 and 16 mm and I'm sure one of them will be the one.