You found the unison seats. I think they came out first in 54 for Chevy and were found more in bigger trucks. The driver's seat has a vent in the bottom of it and it is designed for more comfort on long hauls. Do a search on this site for unison seats and you will find more info.
By any chance was the truck blue? There was a 55 1st series that was restored close to here by a guy named Glen Rusch and Gael Epp. Glen said it was "the world's most perfectly restored 55 1st". Last I heard, he said it was in a museum on the west coast.
