Engineered specifically for the Fujifilm X100T, with ideal length and curve geometry.
Though you could use the X100T thumbrest on the X100F camera, it’s not optimal for most users.
Because the X100F rear control dial is farther from the hot shoe than the X100T, the X100Ts grip cutout isn’t centered over the control dial resulting in too little clearance for larger thumbs to rotate the dial. The shutter speed/iso dial is also difficult to use for larger hands. The X100Fs hinged design makes it easy to access both of these controls. Additionally, the X100Fs larger grip improves comfort with heavier lenses.
See difference between the X100F, the first photo and the X100T below.