The back has a couple of curved tabs that fit into slotted brackets on the rear edge of the seat portion.
Try lifting up on the back while pulling up on front edge of seat. Underside of seat frame has a few holes ( allows adjustment ) Metal frame around gas tank supports 4 brackets with raised pins to position seat.
A bit tricky with one person - best with 2


1940 Chevy KC 1/2-Ton
In the Stovebolt Gallery