Forum,
For the record:
1. With the body wiring harness disconnected from the tail lamp assembly wiring harness > To remove the tail lamp assembly wiring harness connector that is clipped to the assembly: As you look at the lamp assembly itself, the connector is just below and inboard of the stop/tail lamp socket. Lift the left/outboard side of the connector towards you. The connector then slides to the left and is separated from the lamp assembly. After removing the 3 sockets from the assembly, the harness can be removed form the car vice the entire tail lamp assembly. Troubleshooting and repair of the harness and/or sockets is SIGNIFICANTLY EASIER when you can remove it from the car. Plus you won't have to disassemble the cargo compartment (Roadster) to access the 3rd tail lamp assembly mount (to remove the entire lamp assembly).
2. As for the stop/tail lamp socket itself: The following were stamped on the socket CEMM as well as PA66. Even Google couldn't cross these to anything relevant. Short of a visual comparison @ the NAPA (etc.) counter, sourcing an alternative non OEM replacement socket won't prove easy.
3. Ultimately after voltage verification check (with socket disconnected from body harness and connected to same) and continuity checks of the socket, I sprayed all sockets and connector out with isopropyl alcohol and using a jewelers straight slot screwdriver adjusted the 12 vdc contacts and gently pried the ground contact away from the socket sidewall slightly.
4. Passenger tail lamp works as advertised.