I have a picture of what appears to be an original seat back wood assembly. I can't remember were I got it (it is not my 28 roadster, which is missing the assembly). I can email you the picture of it if you send me your email address. Basically, there is a horizontal piece of wood that bolts to the top of the seat riser. The sloping vertical pieces (3 for the seat back) attach to the horizontal piece at the bottom.
Rusty Nelson
Lakeville, MN