To the best of my knowledge, if the Unity spotlight was purchased through GM, 60-66 Chevy trucks would have received the short black GM specific handle. GMC's received the same GM short handle, although it was Ivory in color. The standard "full length" ivory handle would have been supplied if the Unity spotlight was purchased outside the GM channel. I never saw a full size black standard Unity handle until the early 1970’s so I’m assuming everything prior was only produced in ivory. The only exceptions I’m aware of are the custom GM short handles, and some early 60’s Ford OEM applications that were black with a chrome bottom cap.
Here’s a comparison between a short GM and standard full size handle, and a photo of two short GM handles on new and old housings:
I had one photo of the ultra rare GMC short ivory handle, but apparently it was lost in a hard drive crash a while back.