Think about how they would do the installs at the factory. It would be very odd for them to hand a separate gasket to the worker, have him stick that over the holes on the tub and then attach the tail light. Far easier and faster would be to have the gasket already applied and stuck to the housing so it winds up in the correct spot as fast and accurately as possible. Not saying I know they did it that way, but all the gaskets I've seen stuck to tail lights sure point that way.
I can confirm that brand new OEM Mopar tail lights come with the gaskets on them. That is definitely the design intent. Odd that his would have been stuck to the tub...probably previous owner tomfoolery.
