Here are two of my vintage/classic bike tail lights, both “unswitched” – the first is unbranded, made of a thermosetting plastic that looks a bit like black bakelite. I believe it was Australian made but I’m not certain. It is about as simple as you can get, relying on screwing in the cap to make contact between the bulb and “D” cell battery via copper tracks that mate at a certain point of rotation. This light is to be fitted to my old speedwell bike, and has polished up nicely with metal polishing paste on all external parts.
The second is the Eveready, made in the UK, of aluminium. This one is slightly more robust in build and the lens doubles as a rear reflector. I may be fitting it to my loop frame Speedwell when it is finished – it needs a tidy-up, as the alloy has oxidised in places.
The main disadvantage with both these lights is that the large battery tends to rattle around when the light is off and may also turn on at times because of this. Some tweaking with rubber washers should help with this issue. Alternatively, I can remove the battery till needed as these bikes won’t get a lot of night use.
The “D” cells back then must have been mostly insulated on the top (+ve) terminal as the holding spring will short out the modern “D” cells via the terminal plate, unless this part is insulated.