The fixed passenger seat frame was unique, you cannot add seat tracks
onto one, the frame is straight on a fixed seat and dished in on a adjustable
seat for seat track clearance.
Special studs and cups were used to mount them in the car.
The early drivers seats used along side the fixed passenger seat were also
different, the mounting holes for the tracks to the frame are spaced different
from the later ones so the later tracks would not bolt on with modification.