Ric,
Am I correct in thinking that the Mystery Debris was not recovered from the island? These two pieces are heavy, and the transportation costs could be significant.
A piece of steel this size weighs around 250 pounds / 115 kg. (36 x 48 inches, x .5 = 864 cubic inches; a cubic foot is 1728 cubic inches, so this is one half of a cubic foot. A cubic foot of steel weighs
about 500 pounds.) The rivet holes would reduce the weight somewhat, and the measurements are approximate, but that should be a decent estimate.
Some other questions, assuming for the moment that these pieces are from the Norwich City:
Could the fire on board the Norwich City have damaged the rivets so that they deteriorated more quickly than the rest of the piece? And a related question, is there any evidence the piece was exposed to fire?
Is the pattern of rivets - double rows of rivets at the edge, and a single row within the piece - characteristic of any particular part of the ship?
It took a lot of force to warp the metal into its current shape. Could this piece be from a section of the NC that was twisted when the stern broke off?
An additional possibility, and this might be closer to wild speculation: Could these pieces have been used as counterweights on a pulley system, for hauling equipment or people up the tower?