One of the more unusual but little known Supermarine Spitfire variants was the Mk.IIC (later redesignated ASR.II) of which there were fifty used in the Second World War. The ASR.II were specialist air-sea rescue Spitfires converted from retired Spitfire Mk.IIs when production switched over the more powerful and capable Mk.V. The conversions took place from 1942 to 1943 and involved removal of the armament and chutes were added in the lower fuselage aft of the cockpit that could drop rescue packs that consisted of a dinghy and rations, smoke bombs to mark the location of downed pilots and flares. Additional packs could be carried on underwing racks as well.
In addition to these provisions, the Rolls-Royce Merlin XII that was standard for the Mk.II variant was swapped out with a more powerful 1,460-horsepower Merlin XX that had a 2-speed/1-stage supercharger (the Merlin XII developed 1,175-horsepower and had a 1-speed/1-stage supercharger).
The Spitfire ASR.II served with five search and rescue squadrons that operated over the Thames Estuary and the English Channel during the war.
Source: Spitfire by Stewart Wilson, Sovereign Series #1. Aerospace Publications, 1999, p28.