Yesterday I started working on the seat backs. They are made up of these skins, reinforced by angles.

Here the are the 4 vertical stiffeners cut to lenght.

At the upper ends, the stiffeners gets rounded like this:

..and at the lower ends they get angled like this:

At the lower ends, a recess is also needed to make room for the hinge.

Like this:

 After measuring out the rivet spacing, drilling and match drilling, the seatbacks are slowly coming together. Still need the horizontal stiffeners and upper hinge parts.