This is the lower rear pan below bumper and rear body panel. I think all Buick and Cadillac would fit. It is a 2 piece steel part with provision for exhaust to run through right side.
This is a guess. But most headlights were aimed to go slightly to the right and I wonder if this is the reason they are the same. I would say they are correct Also if they are not correct for your car I would be interested in the reflectors. I have a 39 Cadillac /LaSalle that was converted also.