On very rare occasions, a manufacturer would design and build an aircraft using their own money as a "private venture" (PV).
The system was applied to commercial aircraft as well, two being the de Havilland Comet and Vickers Viscount.
In this case this resulted in several types designed to specifications originally intended to signify the naval Spotting role also being used for other purposes, e.g., S.15/33, resulting in the Blackburn Shark and Fairey Swordfish, the latter aircraft being primarily utilised as a torpedo bomber.
Similarly S.24/37, which produced the Fairey Barracuda, again primarily designed for spotting, the dive bomber/torpedo bomber requirements being regarded as secondary when the specification was issued, but for which roles it was almost exclusively subsequently used, the original spotting requirement having been made obsolete with the introduction of radar.
[1] In addition, some (mostly early) specifications appear to have no letter prefix at all, e.g., 1/21, the Vickers Virginia III.
Sometimes planned variants would be later cancelled leading to 'missing' mark numbers, or the extent of the changes may have justified given the new variant a completely new name, e.g., the Hawker Typhoon II subsequently becoming the Hawker Tempest, or the Avro Lancaster B.IV & B.V entering service as the Avro Lincoln.
In a few cases the same aircraft ordered with differing engines would be allocated separate names for each variant, e.g., Hawker Typhoon and Hawker Tornado, or the Handley Page Hampden and Handley Page Hereford.
Note 2: due to mergers and amalgamations within the UK aircraft industry sometimes the name of the manufacturer changed over time, e.g., English Electric later became part of the British Aircraft Corporation (BAC), so the English Electric Lightning then became the BAC Lightning; the British Aircraft Corporation itself and Hawker Siddeley (HS) then later merged and became British Aerospace, subsequently becoming BAe (now BAE Systems).