Could someone please tell me what the two big-headed black plastic screws in the floor either side of the tunnel behind the seats are for? Thanks. (As you can see the DAPO was seemingly fond of driving on the beach!!)Click to view attachment

Optional center seat belt

Early cars only, the plastic plugs protect the threads where the optional 3rd seatbelt mounts to the floor.

Yes that is 100% correct, noticed them in the -6, It took me a minute to remember that Porsche offered that as an option.

When I was a kid 1970s I remember riding on the 3rd seat. We did not have the seat belt.

LOL, I also remember being 4 point strapped in the back of a Ford Station wagon

Those memories may explain a few things

I bought a middle seat belt for my early car, mounted it and then realized (i'm slow) that a sudden stop would direct my kids face into the gear shift knob. Off it came and sold it on to someone who just wanted it for show.

I thought about adding a harness bar and a full restraint but my kids grew up too fast.

Thanks to all who responded. Another mystery explained!