 Complete the following phrase:  In 1942, the two-stage Merlin 60 aero engine became available for reconnaissance aircraft. The first 15 Spitfires with the new engine were conversions of standard Mk IXs made by the workshops of 1 PRU at RAF Heston. One of the best known operations undertaken by the Mk IX conversion was to provide photographs of the four dams slated to be destroyed by 