Been wanting to add rear lighting for a while, now. My wife followed me home one night, said I had no brake or tail light. Found some red lenses to replace my rear signal lenses, in the process of fabricating new sockets and reflectors to use 1157 dual filament bulbs, and learned of
these. When I'm done, I'll have three tail lights, three brake lights, and the signals will blink.

Hammered out copper reflectors, I'll cut a hole in the middle to accept the brass 1157 sockets, solder in, paint them white.

Red lenses I found at swap meet, fit with just a little filing.
Apparently, the tail light converters are common on trailers. They have them at Autozone for $14. The one at Discount Auto is a huge thing.
If I didn't change the sockets, I could use one of them to have running lights and signals with only one filament.