Yes, the seat back is also bolted to the frame.  You need to pull up the upholstery on either side to access them.  Not sure if there are any in the middle or not...

1956 Belvedere 4-door sedan. I have pulled the front seat seat-bottom up and out of the base frame, and removed the painted trim that was fastened to the base frame. Q - How do I remove the back-rest? I tried pulling it up, but it would not move. Are there screws that hold the upholstered unit to the back frame? Dick Friedman;