A Lancaster Mk I (R5727) from Avro Aircraft (UK) was flown across the Atlantic in August 1942, to act as a "pattern" aircraft for production. Differences between the British Lancasters and the Canadian built versions (known as the Mk X) revolved around engines, instruments and radio equipment being manufactured in Canada or the United States instead of England. The Rolls-Royce Merlin engines were the same design, but manufactured by Packard in the United States. A later series Lancaster Mk X replaced the original Frazer-Nash FN 50 mid-upper turret with a US-made Martin 250CE turret moved to a new location farther back on the upper fuselage due to differences in weight between that unit and the original turret. All major sub-assemblies of the Canadian Lancasters were interchangeable with the British versions so that in the event of damage, spare parts were immediately at hand.