The marking means 20 Liters or 5 Gallons, with a 1951 Date. This is an Army marked Gas Can with a U.
A marking on one side... MC marking on the Other side... Both sides of cans have "G" gas markings.
Army marked gas cans from Quartermaster Corps Q. During the Korean War Army Gas Cans changed to just having a U. S marking on the side. Navy gas cans are marked U.