Don't cut the ring off. We need to see EVERYTHING it says on that old bulb base, not just the little bit in the one picture.
The one with the ring is "american prefocus". We don't want that. A couple of years before american prefocus there were manual focus bulbs that just had a bayonet base. It probably really is the same as some 20s or early 30s car or motorcycle bulb. We can see in the picture that it is a 6v bulb (6-8 volts allows for the charging system in a motorized vehicle). I see "MCP" but no number. MCP should be candlepower. How many candlepower?
Get the number in front of MCP, whatever it is, and go looking for a single-filament bayonet base 6 volt automotive bulb of that many candlepower. You will want something that was intended to be focused, like a headlight, spotlight, or fog light.
When you find one with the same candlepower, you will have a bulb number. Then go online and find a mechanical drawing of the bulb. 1000 bulbs may have that. Tail light king may have that. Don's Bulbs had all that but is gone, although some of the material can still be accessed through the Internet Archive. If you find that the "LCL", a measurement from the top of the bayonet pin to the center of the filament matches, you have found your bulb. If not keep trying bulbs of the correct candlepower until you find it.
