I know about the gate valves and the plywood in the sump that causes the Catalina smile but where is the mild steel and which models have it?
I am not sure about the Catalina's but a number of boats including the Endeavour 33 used unprotected mild steel in the mast step support. Without maintenance it rusted out badly and became a very expensive repair as all the head cabinetry usually needed to be removed to fix it.
I don't think they were the only company to do use steel in the mast step, but I doubt you will see that kind of construction on a more modern boat, an experiment of the time.
Oddly the Endeavour 33 used an entire grid of fiberglass encapsulated steel throughout the sole of the boat. On the one I saw there didn't seem to be any issues with delamination and the boat was almost thirty years old. Go figure.