To add a little to Brad's information, I think that the first year that Chevrolet trucks had a Chevrolet accessory turn-signal set was 1951. This accessory-set had a clamp-on switch and front-fender-mounted and rear-body-mounted lamps. In 1953 the switch was the same but the lamps were a different style (in the same front-fender and rear-body locations as used in 1951). If you want to see photos, let me know.
In 1954/55, GM (Chevrolet trucks and GMC) had more-modern non-clamp-on turn-signal accessory sets for all sizes of trucks. On smaller trucks, the front signal lights were inside the front parking lamp, using a substitute dual-filament bulb/socket. Larger trucks were given a fender-mounted lamp. In 54/55 the accessory rear lamps were
different for different model trucks.
