According to my spreadsheet for 67-68 Mustangs based on the "1968 Mustang Electrical Assembly Manual" (AM0023 pg53), one screw 50514-S36 is used to attach the fog lamp switch AND 381726-S100 CLIP to the underside of the dash with nut 34653-S36, and a second screw and nut are used for the same RELOCATED fog lamp wiring clip used WITH the tilt steering wheel.
Jim
Upon further review, two screw/nut combinations are used WITHOUT the tilt wheel option. With the tilt wheel option, the wiring is moved to the right and the screw/nut that holds the switch is used, probably for tilt column clearance.
Screw 50514-S36 is 1968 unique, used for the convertible top switch and speed control switch, as well as the one or two for the fog lamps. As it is an 8-32X1/2 machine screw, substitutions are probably easy to find from other assembly line uses. Just don't reference this thread if your 68 Mustang is in the Thoroughbred Class.