The Mk III Manchester (serial number "BT308") which first flew on 9 January 1941, was essentially the first Lancaster, featuring a longer wing fitted with four Rolls-Royce Merlins in new unitized, power-egg nacelles – originally developed by Rolls-Royce for the Merlin-powered Beaufighter II – although initially retaining the three fins and twin outboard rudders (the central fin had no movable control surface) of the Manchester I. "BT308" received the "Lancaster" name immediately after its first flight. The second prototype Lancaster "DG595" featured the twin, enlarged fins and rudders of the Manchester IA. Manchester production continued until November of that year but some aircraft that were still in production were instead completed as Lancasters.