Lloyd's of London, the biggest marine underwriter there was for a long time, required in their policies that all merchantmen to be armed. The first thing that was determined in admiralty court in case of the loss of a ship or cargo was the armament of the ship in terms of number of swivels and number of carriage guns. I've read a few records of proceedings of these courts and that's always at the beginning. I seriously doubt if anything with some salvage value would be deep-sixed unless required to save the ship in bad weather.
Insurance guns were used aboard US merchantmen at least up until the Civil War. Many of the guns were English but Cyrus Alger would cast any kind you wanted, any kind of muzzle, any kind of cascabel, rifled, smoothbore, I've seen more different combinations marked Alger than there are species in the National Zoo.
The story you heard sounds like the Napoleon over yonder in the swamp that the Yankees left behind, that you can only see after an extended drought.
