As a bluegrass musician, I am familiar with the song, which was a standard for the group Lonzo and Oscar. ("Many many years ago when I was twenty-three, I was married to a widder who was pretty as could be..."). I have never considered a connection to Mark Twain, but I sure will be astonished if someone can uncover one! (If you work your way through the genealogy of the song, the singer really is his own grandpa...)