Wow! What a great piece! I believe Wayne nailed it with the swivel attribution. It would appear from the pics (hard to tell, though) that the trunnions are set lower on the gun, which they would be if it were truly a 'rail' piece and allowing for steep angling downwards. Brass and bronze guns were more resistant to the salt erosion of sea air. Nice find!! I'd beg on the street for a piece like this in my meager collection-
