W123 1985 - What fits in this hole?

[Graham]

I am in process of putting interior of my 300D back together. It's a bit like a jigsaw puzzle, but without the picture on the box

I so far have just one thing I can't recall. There is a hole with a slot above it on each door silll, just about opposite the rear bolt clip. There is a hole in carpet, so something must have gone in there! There is another hole slightly further back and I assume that is where the seat belts attach. I have the bolts with sleeves etc for there.