Maybe not a treasure, but what is it?

[riverdiver]

I would have to say that it used to hold a fountain pen and the chain was attached to the pen like they used to have at the banks. Fountain pen try would catch the ink that would over run from the tip of the pen.
Ed

 

[vshearer]

It is the cover for an anchor chain/rope pipe. Look at the west marine website for chain/rope pipe and you'll see a picture. These are normally made of either chrome plated brass or stainless steel. It's not unusual for the cover to be pulled off the deck if it's not removed before anchoring.

Bingo. And there is sometimes a hook on it to catch/retrieve the rode if disconnected from the anchor while stowed.