The first versions held sardines and beans; when sardines did not sell, vendors switched to ham and kept adding fillings.
A split, hollowed bolillo layered with black beans, ham, head cheese, chicharrón in green salsa, two cheeses, chorizo, shredded beef, tomato, onion and avocado.
Why it's made this way: It was born around 1928 in Tampico, sold beside the wall ('barda') that separated the railway, docks and customs from the city, to rail and dock workers.