the Spitfire Mk V). Occasionally other 'minor' but nonetheless important changes might be denoted by series numbers, preceded by "Series", "Srs." or "Srs" (e.g. Mosquito B Mk IV Series I / B Mk IV Series II – the different series number denoting the introduction, after a few initial production aircraft, of extended engine nacelles to eliminate buffeting. This design change was made standard on all subsequent production Mosquitoes). The series number denoted a revision during the production run of a particular Mark. This again could then have an additional letter-suffix (e.g. the Halifax Mk II Series IA). Export variants of British military aircraft are usually allocated mark numbers (sometimes without a role prefix) from a higher range of numbers, usually starting at Mark 50. A converse convention was adopted for the Canadian-designed de Havilland Canada DHC-1 Chipmunk, where the sole British service variant was designated Chipmunk T.10. Up until the end of 1942, the RAF always used Roman numerals for mark numbers. 1943–1948 was a transition period during which new aircraft entering service were given Arabic numerals for mark numbers but older aircraft retained their Roman numerals. From 1948 onwards, Arabic numerals were used exclusively. Thus, the Spitfire PR Mk XIX became the PR Mk 19 after 1948. With this change, the Sea- variants were allocated their own range within one common series for all variants (e.g. the Hawker Fury Mk I was followed by the Sea Fury F.10, Sea Fury FB.11 etc.). Format of designation The system has been largely unchanged since 1948 with the addition of more prefixes as new roles have arisen. For example, the first Lockheed Hercules variant in RAF service was the Hercules C.1 ("Cargo, Mark 1"). A single example was adapted for weather monitoring purposes and became the Hercules W.2. The stretched variant became the Hercules C.3. With aircraft with a long service life, as their function changes over time, the designation letters and sometimes the mark digit will change to reflect this. The practice of restarting the mark numbers for the naval variant where the name was changed continued – e.g. the naval version of the Harrier, the Sea Harrier, marks started again at FRS Mk 1 – whilst variants where the name was unchanged for the naval version such as the Lynx have a single set of numbers for both land and naval variants. In the case of the Sea King, which