A heavy canvas sack of the size a porter shoulders and a miller reckons in. Fifty pounds is about as much as one person carries any distance, so the sack is a measure as much as a container — dockers and millers are paid by it. The bottom seam is doubled, since that is where a full sack always goes.
Sack, canvas, 50 lb
Heavy canvas sack sized for a porter's load (50 lb).