Hello Ralf,
Perhaps a bit late? But here some pictures of my rear seat cushion(fastback).
I can only provide the pictures of what I have dismantled.
I haven't had the time for rebuilding the rear seat, even all parts are ready to put them together.
(the seat frame has been soda blasted and repainted), bought new burlap, the cloth strips are reusable and new hog rings)
If you're in need of some details, just ask.
Kind regards,
Jeroen