There is this from Wikipedia....."The Holman Projector was an anti-aircraft weapon used by the Royal Navy during World War II, primarily between early 1940 and late 1941. The weapon was proposed and designed by Holmans, a machine tool manufacturer based at Camborne, Cornwall. A number of models were produced during the war years, but all worked on the principle of a pneumatic mortar, using compressed air or high pressure steam to fire an explosive projectile at enemy aircraft." I hope this is useful to you.