Here's a heavy cast iron radiator cap that fits the Model T radiator. I would think this was used on a Model T stationary engine set up since it's awfully heavy for the radiator neck of a road vehicle that bounces around on the open road.
I recently baught a pile of T stuff and in it was a different style cast iron cap I'll try to post a pic.
At a show in Rhineback NY this past year I saw one that was Henry Fords head in cast iron but I didn't buy it for $2500.00
I have a couple of cast iron dog bone caps.
There is a vent hole in that cap - sure it is not for a gas tank??