Arlo is correct, there is a spacer piece the mounts to the bulkhead behind the seat. The PU top extends beyond the bulkhead and this part fills the gap and supports the back of the PU top. This part is NLA and can be very difficult to find. I saw the seatbelt mounts for sale somewhere recently so they should be available or may come with a PU top.
THING 1 - 1973 88 SIII - SOLD
THING 2 -1974 88 SIII Daily Driver - SOLD
THING 3 - 1969 88 SIIA Bugeye Project
THING 4 - 1971 109 SIIA ExMod - SOLD
THING 5 - 1958 109 PU
THING 6 - 1954 86" HT