This IS the greatest of the broadside ballads - too bad no broadside's ever been discovered!

Novelist Jack London reported learning the song in the 1890s around San Francisco Bay. This would seem to be about the time that it was so popular in lumber camps throughout the north. That might suggest an origin in the 1880s or a bit earlier: just a thought.

Most people don't realize that it was once usual to anglicize the sounds of any and all foreign words in an English context. (The pronunciation of Latin in legal contexts may be the last remaining example of this.) That said, I've always inmagined that the Spanish man-o'-war the "Dungeon" simply rationalized a mishearing of Spanish "Don Juan" (Don-Joo-Un). Lord Byron once used this very rhyme. (Of course it might just as easily have rationalized an unfamiliar "Dungeness"!)

FWIW, I've never come across any "keys" to the "mystery of the Flying Cloud" in decades of keeping my eyes open.

But don't let that stop you!