On all models the tail light housings were held to the body via a threaded post cast into the plastic light housing. The threaded post passes through the body sheet metal and is retained by a small 8mm nut. It appears you have a 79 or later style tail light. That aluminum retainer that you see is commonly seen on the 78 and earlier tail lights to retain the lenses to the tail light housing.
At some point in the history of your car either the small 8mm nut was lost, or the PO got fed up trying to get the nut on/off since you need a deep well socket. So....the small 8mm nut was replaced with that aluminum retainer...which also works and is probably easier to remove than the small 8mm nut. Just grab it with some pliers and twist it off, but be careful since almost all the tail light threaded posts that are cast into the tail light housings are usually corroded and will sometimes twist off or pull out of the tail light housing.
I did try to take off with bare hands but skin came off my fingers instead. Have also been trying to remove it using pliers with little luck. Can twist with pliers but retainer never really loosens up. Retainer also doesn't look like it has backed out at all. Even after initially spraying the outside and down the hole with WD-40.
Guess I'll take the other nuts off first and mess with more.
If it doesn't come off I am tempted to write SpaceShipOne suggesting they consider using Fiat aluminum retainers on their suborbital space vehicles as a permanent connector.
If it's turning but not comming off, my guess is that the stud in the tail light Iis spinning, although the threads in the aluminum nut could be stripped. Usually the studs just break off for me and I knock them out and replace them with rivnuts and bolts.
I have the same thing on my 76 and it was a bear to get off. Use a lot of WD40 and take a 3 day vacation and when you get back it should be loose enough to remove.
Those long aluminum nuts were to hold the plastic inside the trunk tail light cover (which most of us are missing) Originally those plastic covers kept stuff out of the tail light assembly, but were a pain in the neck every time you had to change a bulb. The small nut are what hold the tail light in place, the aluminum long ones are for the plastic coer only if memory serves me correctly. New style lights 79 and up didnt have the plastic covers.
Michael and Deborah Williamson
1971 Spider -Tropie’ - w screaming IDFs
1971 Spider - Vesper -scrapped
1979 Spider - Seraphina - our son's car now sold
1972 Spider - Tortellini- our son's current