I found this large calibre 1943 dated cartridge at Buckland Paper Mill last Friday whilst helping clear rubbish out of the Dour with the White Cliffs Countryside Partnership. It is marked "SL 43" and is of US origin. The smaller cartridge is a .303 rifle round I found about a meter away.
I've tracked it down and believe it to be a .50 calibre round as used in Browning heavy machine guns, the standard armament of US fighters and bombers of that period. Strangely, it appears to be unfired. Any theories chaps?