INTERESTING. BUT DIFFICULT TO FIGURE.
ON SHIPS ARMS WERE MOSTLY LOCKED IN THE ARMS LOCKER. PERHAPS A CAPTIAN MIGHT HAVE SOMETHING LIKE THIS IN HIS CABIN TO SECURE A GUN WHILE HE WAS OUT OF HIS CABIN. THE THING WOULD NEED TO BE ATTACHED TO SOMETHING SO IT COULD NOT BE CARRIED OFF. IT DOSEN'T CONFORM TO OLD LONG BARRELED PISTOLS TOO SHALLOW AND TOO BIG FOR A NORMAL GUN BARREL. PERHAPS FOR A FLARE GUN??

GOOD LUCK FINDING MORE INFO.