It looks like the truck windshield washer was "Push-button start; atutomatic operation and shut off." from 1947-1950.
From 1951-1953, the washer was foot-activated (a button, as shown in the 1951 Accessory Installation Manual, linked above).
In 1954/55, there was a foot-activated lever that not only activated the washer, but there was also a cable from the foot-level that turned on the windshield wipers. That is why the 1954/55 windshield-wiper motor is a different part number from earlier years.
