'59s all had the 'cat eye' taillights. Here's a perhaps better example, the Pontiac Banshee. Not a tacked-on feature, it's almost an external duct.
https://www.pontiactransamforum.com/...t-car.571/full
I'm thinking of the groove that starts in the middle of the door that feeds into a cavity. I have
no idea about the airflow around that feature.