Well, I hope there is not too much significance to this, but some research shows that the USS Reina Mercedes was in the Spanish Navy, but captured by the US in Cuba during the Spanish-American War (1898). She was subsequently raised and served a variety of purposes. For a period of time (until 1940) it appears that she served as an informal "brig" for US Naval Academy Midshipmen in Annapolis Maryland. Perhaps Grandpa picked this up during one of his "visits" to the ship ... I'll never know.
