Since these posts remain archived forever, I need to add the following info to my previous post, which I did from memory (I didn't have access to my parts stash at that moment). I found two different new clearance/cab lights: (1) Autolamp Pathfinder model 472, teardrop shape, full-length amber plastic lens marked Autolamp 472 SAE 60. The base is aluminum. The box is marked "State approved" which makes sense as the 1960 date pre-dates federal regs. (2) Yankee Fleet Products model 77, teardrop shape, front half amber plastic lens marked Yankee 77. The base is chrome-plated pot metal marked Yankee 77 SAE-P-66. The box says it conforms to FMVSS 108 (Federal Motor Vehicle Safety Standard 108, which covers all lighting, and was new at the time).
The federal requirements rely heavily on the standards established by the SAE, which is why the 1960 lamp carries the SAE lettering. Not sure when the lamp manufacturers started complying with the SAE standards.
I'm still trying to find any info on the appearance or source of the clearance lights offered by Studebaker as accessories on 2R and later trucks. They probably were very similar to one of the above. Grote and Dietz were also big makers of truck lights. Plastic probably replaced glass lenses around the same time it did in car tail light lenses, and not as late as I implied in my earlier post.