"I still believe it dates back before 1856: see below
From BBC program about antiques: The British war ships of the time of Nelson and Trafalga had square plates to fit the tables slung between the cannons below decks. So many sailors were from such…"
"Ye Olde English Sayings
GETTING A SQUARE MEAL ... A square plate allowed a larger amount of foot on a relatively smaller plate. So a square meal was a larger meal than they would otherwise be having - a good square meal being a favourable thing."