What is this nut/bolt?

What is the reinforcing plate…thats on the RH side just before where the fuel line attaches to the aft bulkhead under the car ,it has a bolt from the top and the nut is welded to the plate,cannot find it from the top picture will not upload

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Here is a picture

Seat belt anchor point?

If it’s not the seat belt (I suspect it is) maybe it reinforces the drop top pivot bracket?

It you look at it…it is very high on a small ledge… too high for a seat belt…there is an identical one on the left side…but mine on the left seems to have a plastic plug in it…

Look at the orientation to the fuel line…its on a ledge .way back from the top piviot …there is nothing there that i can think of

The shoulder belt attachment point on OTS cars is on the flat ledge behind the seats and from the factory they were plugged up with plastic plugs to avoid splashed water from the rear wheels getting inside.

Those holes on my late series 2 OTS have a round disc spotwelded on the outside to close them off.

Here are the holes on the inside.

Closeup of the inside right side.

Very odd, it would be interesting to know the purpose. With your car being an early series 1, perhaps it was some kind of prototype feature.