55-57 Thin back.
Seat backrest has small coil springs woven into the spring pad.
58-59 thick back.
Seat backrest has new zigzag spring design in the spring pad.
Extended lower backrest frame 57-59, Lower flattened backrest frame 59-67, Smaller frame strengtheners 57-59, Larger frame strengtheners 59-67.
59-67 Thick back.
Seat backrest has 3 horizontal bars at the back, separate back panel is secured to these bars.
The seat frame on the 57-59 model was this much longer at the back, on the 59-67 model an extra long strengthening plate was inserted to redirect all the backrest pressure, distributing it further up the cushion frame stopping the frame from fracturing.
Depending on the interior colour, the seat frames from 57-60 were mostly silver beige.