Seat backrest springs

[1930.68B]

We are restoring a 1930 Cabriolet with a screw type adjustable seat. Synder's list the backrest springs as 68B (S-306) and 68C (S-306-L). Some early 1930 Cabriolets had a fixed seat that carried over 1929 that is not listed in any of the parts catalogs for a 1930.
Am I correct in assuming that Synder's destination between 68B and 68C is the difference between a screw type and ratchet type adjustable seat?
The height of our seat frame is 24" (photo attached). Is the dimension different on the ratchet type seat? Lebaron Bonney request this dimension on their form for upholstery.