At the time N.4/43 was written, the Seafire F.XV prototypes NS487, NS490, and NS493, based on the Supermarine Type 377/386, were already ordered through Contract Aircraft/2901 on March 8, 1943. Additionally, three Mk XV pre-production aircraft, PK240, PK243, and PK245, were also ordered through the same contract on May 25, 1943. These six aircraft were the only Mk XVs constructed by Supermarine, as production orders for the XV were placed with the established Seafire manufacturers, Cunliffe-Owen at Eastleigh and Westland at Yeovil. The first Yeovil-built plane was delivered on August 31, 1944, and the first Cunliffe-Owen example was supplied in March 1945. However, the end of the war led to a significant reduction in Mk XV orders, with only 433 fighters eventually built by Cunliffe-Owen and Westland, despite initial orders approaching 1,000 machines.