These seem to be hard to come across,took me quite a while to find one..they were mainly given out to the airforce to protect the airbase from being sabotaged and a small handful were given out to the navy..I was told this one was used in Halifax at the end of the war by Navy boarding parties where many german Uboats were turning themselves over...the same guy told me the reason for the 50 round mag was that nobody in the Airforce or the Navy could shoot straight,hence the extra rounds