I hesitated before posting this - in some cultures it is risky correcting a Grand Poohbah.
In 1954, complete turnsignal kits were available from GM as an accessory for all sizes of trucks (different kits for different trucks). A link to a photo of these accessories is given
in a post above. Also, fender-mounted directional signal lamps, but not the controller, were available as GM accessories for earlier Advance-Design Chevrolet trucks.
Here is page from the 1949 Chevrolet Truck Data Book. Tim