The four door sedan is suppose to have the additional piece included on the
front door door sills that have the Imperial script just like in '55.
The Southampton four door hardtop did not include this part on the actual cars,
although I believe that it is pictured in the sales brochures. I added it to
mine from a sedan parts car years ago, but later removed it when I needed the
script for the right front fender of one of my '55s.
I have seen Southamptons both with and without the additional "script piece".
It has been included on all of the sedans that I have seen.
Paul