It appeared that, after the bulbs were installed, the assembly was sprayed with some sort of clear lacquer before the lense was installed. Does anyone know what the spray was?
This is a guess, but I would suspect that the coating that you are seeing may very well be PlastiCoat. If the bases were installed prior to the final finishing, which I suspect that they may well have been, it would almost be a natural to have PlastiCoat on the exposed surfaces. With the protection afforded by the lenses, the PlastiCoat would likely survive unscathed for many years if that is the coating.
Since I am currently servicing the turn signals, backup, and marker lights, what do you recommend for "care and feeding" of the electrical contacts?
Dr. Jonhnson, an electrical engineer on another list to which I subscribe, recommends dielectric grease on the electrical contacts as well as ground contacts. He also suggests aluminum conductor grease on the ground contacts that touch the skin of the trailer.
I haven't disassembled the lights on my trailer, but I do use the the dielectric grease that Dr. Johnson suggested and it has made changing bulbs much easier.
Good luck with your project!