What's this contraption?

The tubular steel thing, bolted in where the bench seats would be?

Iirc , from removing mine. Part of the seatbelt retaining system.

Cheers

Yes, that was once a CSW with four inward facing seats. That's the tubular steel bar that the seat belts bolts to. Underneath, at the back of the wheel arch, there is a tubular bracket that connects it to the rear cross member. That way the seat belts are directly bolted to the chassis.

Edit,
suppose it still is a CSW!