The lower plate you pictured was part of a discussion a few years ago. Mine doesn't have one and I don't remember either of the seats in my first '55 having one. I don't believe there was a consensus on which trucks/years/makes had the seat that you pictured. There was an "air cushion" seat available as an option but that one actually had bladders under the seat instead of springs.
http://chevy.oldcarmanualproject.com...55ctsm0113.htm
I have never seen one in person but I know someone on the forum posted pictures of one a year or two ago.
The seat in my '57 had some broken sprngs when it was disassembled. I didn't do that job though. The interior shop I used in MT told me they replaced the springs. They also welded up a couple of broken supports as well. My seat came from a truck at the junkyard so it's no surprise it had some damage. It rode like an old truck seat before it was repaired and it rides like an old truck seat now. At least the cloth cover is much nicer than the old vinyl.