I'm not sure any clearance lights are "original " in that the GM truck rolled off the assembly line with them? Were they dealer installed? At some point/year GM did install lights new, but I'm thinking this was after the 50's?
The light you pictured is a KD 517, it is old enough to be on the truck from about new. That's my favorite model clearance light, nice heavy glass lens and good style. I've collects couple sets of NOS ones for my trucks and some spare lenses, which came red, amber, and clear. I might still need one more clear.
Grigg
