Sidevalve,
The rumble seat did not use any spring frames. The two upholstered seat springs fasten directly to the trunk platform and to the lid. If you are referring to the front cushion, it does use a wood frame. I built a new frame for my 28 special coupe (same frame as a 29 sport coupe) from an original (rough) frame a friend gave me. You can order the frames from most model A parts places for $90 to $100 plus shipping. I bought one and returned it as the quality was not a good as I wanted. Hope this helps.
Rusty Nelson