A finished hex jam nut in metric is made to DIN 439. It is about 2/3 the height of a finished hex nut but is identical in every other way. They are designed to go onto a nut or screw with matching threads. They are commonly double chamfered (that's the little bevel you see on the outside of the corners, to help avoid sharp edges damaging finished product) and double countersunk (that's the ring you see on the inside above the thread, to help the fastener find its way into the hole). They are often used to "jam" a second nut against to form a sort of lock, hence the name.