A few months ago I came across a tail light housing that has a lens for illuminating the license plate. I
posted a thread on Stovebolt and got some valuable info from guys who had seen it before. In a nutshell, the housing has a clear lens along the bottom surface so that the tail lamp bulb lights the license plate in addition to the tail lamp. Apparently this was fairly common in the mid-50s across many brands. It was used usually when a vehicle had only one tail light, rather than a left and right installation.
Maybe your connection works better than mine. I tried to verify access to the photos that I linked to in the thread, and I get part-way through, but then I time out. (No love lost with Yahoo shutting down down their Photos "service.")
I doubt you could make use of such a lamp since it forces you to move your license plate installation far outboard. Mine would have been a tripping hazard.
Just another data point to consider.
Bill