Complex sentence: During the immediate post-war service, the Fleet Air Arm quickly replaced its Merlin-powered Seafires with Griffon-powered counterparts, initially adopting the Seafire Mk XV and Mk 17 and, from 1948, by the definitive Seafire Mk 47.
Split into simpler sentences: During the immediate post-war service, the Fleet Air Arm quickly replaced its Merlin-powered Seafires with Griffon-powered counterparts. Accordingly, the service initially adopted the Seafire Mk XV and Mk 17; from 1948, the FAA began accepting the definitive model of the Seafire, the Mk 47.