My experience is that the terminal Fs (F-122, 4704, Unsaver) have spacebars physically similar to standar M but use thinner stabilizer wires that attach through holes in the spacebar underside rather than "C" shaped clips, and the hold-downs pressed into the front plate are much smaller with narrower mouths. The wires are also shorter, the M wires slide up very close to the base of the barrels when full depressed.