The switch doesn't have a jam nut under the bracket, unless it was placed there afterwards. As in your picture, there is a plastic nut on the switch that has a skirt on it which presses thru the bracket, and the hex lays on top of the bracket. This allows one to hold the switch and rotate the nut with its skirt to adjust the switch.
My repro from Yamiya actually has a knurled thumbwheel rather than a hex.
While we're at it, this is the brake pedal return spring which is mounted on the brake pedal pivot on the inboard side of the frame:
Carry on.