Phil,
While this may not help with the size of the seat, I have an enclosed model D. parts list from the fifties that shows the seat and 'fixings', complete with prices of all parts at that time. The fixings are listed as SL 22 plate, seat ball catch---SL 24 seat clip backing strip------SL 25 seat clip packing strip.( I have never owned a D as the parts list is a far as I got, I then bought a C).
6 pounds would have bought a new seat, although at that time may have been most of a weekly wage. The page can be scanned full size and emailed to you if required.
Col.
While this may not help with the size of the seat, I have an enclosed model D. parts list from the fifties that shows the seat and 'fixings', complete with prices of all parts at that time. The fixings are listed as SL 22 plate, seat ball catch---SL 24 seat clip backing strip------SL 25 seat clip packing strip.( I have never owned a D as the parts list is a far as I got, I then bought a C).
6 pounds would have bought a new seat, although at that time may have been most of a weekly wage. The page can be scanned full size and emailed to you if required.
Col.