Machine Screws, Brass - 0BA, 1BA, 2BA, 4BA - ROUND/FLAT, slotted head
0BA, 1BA, 2BA and 4BA
Two head types round (aka dome) and flat (aka countersunk)
note: flat machine screws are measured from the top of the head to the tip. dome/hex/slot/Allen head screws and bolts, although may be flat on the top and the bottom, are measured from the bottom of the head.