Is it wired to anything?
A previous owner could have installed it for some reason: a backup starter button for a worn out ignition switch, an optional horn switch, etc...
Pedal operated washers were "manual" using the pumping of a bellows. The upgraded accessory included an electric pump. I'm not sure, but would guess this option was only available on perhaps the last one or two years of the body style ('65 & '66). My dad had a '66 with such.
Even though my 63 was a Custom Cab, it was fairly basic. It had no w/s washer, two speed wipers, no heater but had the vent knobs, one arm rest, one visor and a cool zippered door pocket.
It had a V8 and three speed with o/d.
I kind of wish I still had that truck.