Hi Ken,
Thanks for the information which adds to the story Ken. It is interesting that the tie wires led to improvement but no mention of coil springs.
The donor springs I got came from a car that I know its history relatively well (FC Special). I am fairly confident it was not re-upholstered. So may be the dealer had them put in. When I removed the coil springs there was / is a "notch" in the frame where the springs clicked into. See an image below of a FB / EK (or is it EK as your FB Picture doesn't show coils) frame and you can see the coil springs and the "notches".
As highlighted by your Service Bulletin, I wonder if there is a difference between the variants ie: Special being more luxurious got an upgrade from the FE to FC.
Then again I am not sure now. I have just looked in the master parts book and it seems the Standard Sedan and Station Sedan had the same part number, the Special Sedan and the Special Station Sedan had the same number, the Business Sedan had the same number and the Ute and Panel Van had the same number. The part numbers are for both FE and FC.
Things changed slightly with the FB/EK. All variants in FB/EK had the same part number except for the Panel Van and Ute which had the same number.
One again thanks for your insight Ken as I think your explanation is the most plausable other than the notches in the frame. If I get a chance this coming weekend I might dismantle the FE seat and investigate more.
Cheers
Rod