Purpose of Convertible Top Well Liner

I have the same style seats in my '68, and the shoulder belts attach to the rear seatbelt mounts near the trans tunnel. They have a quick disconnect feature in case someone needs to sit back there. I bought them from the place I sourced the seats. They make for a nice restraint system consisting of two shoulder belts through the seat with a single strap to the rear seatbelt mount and a lap belt.

I have a rollbar too, but it is only a four point without the crossbar, so my back seat is still functional.