Sealing that is a good idea. Apparently, there can be hot air "backwashed" from the radiator that can find its way into the cabin via the backbone. My car also had a slab of foam rubber that lived under the elbow pad. I realized that one purpose of that is also to keep hot air in the backbone from coming into the cabin.
Yeah...
On the Type 54, the front T section of the chassis has two large central holes - one on the front face and another on the rear face. Presumably all models are the same/similar.
My car had the large rubber bung in the front hole but nothing in the rear one adjacent to the backbone. I don't know if the factory installed a bung in the rear hole but that seems the obvious remedy for the hot air "backwash".
Did we all only get one bung, or was it just me?