giving a GUESS on what i think happened.

if it was K car based they made chrysler lebarons and dodge 600 converts from the factory. these cars were originally built as hard tops at the factory then sent out to be converted to convertibles.

what you are thinking COULD be something the dealer converted himself.
when i worked at a Chrysler dealer before the factory made K car converts. they sent out 5 lebarons and had them converted them to convertibles before the factory did this.

i also personally installed factory accessory ground effects, ft & r bumpers with lower rocker panel trim, and tinted headlight covers on about 20 plymouth turismos and put factory 69 GTX name plates on the 1/4 window panels. i painted the entire car a contrasting color from the body side molding down also. the cars actually sold pretty quickly. they originally were going to just a few cars. they sold quickly and customers requested this be done on cars to their preference of colors. i probably would have done more but the parts department ran out of the accessories. by the time they got new parts the customers didn't want cars like that anymore so they stopped doing this. i think the last one i did sat on the lot for a LONG time before it sold.

so it's possible the dealer converted a k car to a convert and put duster name plates on them.