70 Mustang Coupe cant identify missing part

Hello all. Im new to to the forums. My dad and i have been restoring a 1970 mustang coupe over the past year. Piecing her together part by part has been a real challenge but admittedly it has been very fun and rewarding. We, my father and i have built and put this car together our selves down to the paint job. But enough rambling. I came here to ask a question about a missing part that i cant identify.

What goes here between the door jamb post and the inside rear quarter panel? There is a gap.