I believe it is a dual contact bulb socket.
If it's socket is a two contact bulb socket, it's a tail/license and stop. If single contact bulb socket, it's a tail/license light.
1. For dual contact socket: The less bright filament is just spliced into tail lights. The bright filament must have a separate wire from brake switch. (you don't have to use it has a brake light even though it has a dual socket).
2. For single contact, splice to tail lights


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