I'm not familiar with '46 trucks, but on the later trucks (47-55.1), the seat back cushion is held in place on top with two brackets and screws at the top of the seat frame. Take a look up there.
[on edit] See the attached pic of the seat back on my '51.
Thanks klhansen, but the 46 doesn’t appear to have that loop and bolt. There is no exposed bar around the 46 seat.