The right hand end bolts to the rear chassis side to side brace, to which the top of the shock absorbers bolt.
The square plate has a matching square plate which bolts on either side of the fibreglass side of the engine compartment, (about where the brake servo goes on the left side).
That matching square plate has a similar metal rod which attaches to the diagonal and cross braces on the fire wall, and the top of the seat belt then bolts to it.
In effect the two rods connected by the plates, take the load from the seatbelt and connects it to the back of the chassis.