The word mark, followed by number, is a method of designating a version of a product. It is often abbreviated as Mk or M. This use of the word possibly originates from the use of physical marks made to measure height or progress. Furthermore, by metonymy the word mark is used to note a defined level of development or a model number.
The kind of products that use this convention vary widely in complexity. The concept shares some similarities with the type designation (in hardware), also called software versioning: 1.0+ (1.1, 1.12, 2.0, 3.0, etc.), used to designate general software product releases, and other version control schemas. Thus designations like "Mark I", "Mark II", "Mark III", "Mark IV", etc. come to be used as proper names for persons and products.
Mark refers to a mark on the modification plate of a system, component or machine. Modification plates are used to identify which modifications have already been applied to the device, either at the factory or by maintainers. The use of Mark as a method of versioning has entered common usage however, and may be applied to devices without a modification plate to physically mark.
In British military practice, Mark ("Mk") designations were given in Roman numerals (replaced by Arabic numerals in 1944) to reflect variants of or production changes to service weaponry, either on their own or as part of numerical ("No.") designations; in the Lee-Enfield rifle series for example, the SMLE rifles were produced to Mk I, Mk III, and Mk V specification (with the latter two later gaining a numerical designation of "No. 1 Mk III/Mk V Rifle"), while the No .4 rifles were produced to No. 4 Mk I and No. 4 Mk 2 specification. Variations or production changes could be further denoted with an asterix as with the SMLE Mk III* (later "No. 1 Mk III*") and No. 4 Mk I*. The British military switched to Land Service or "L" number ("LXA1/A2/A3/etc.") designations in 1954,[1] but items such as the No. 8 Mk 1 0.22in Rifle,[2] [3] [4] No. 80 Mk 1 White Phosphorus Smoke Hand Grenade,[5] No. 1 Mk 3 6 Inch Beehive Demolition Charge,[6] [7] and No. 14 Mk 1 11 lb Hayrick Demolition Charge[8] continued to be referred to by their numerical designations until replacement.
British railway coaching stock has been referred to using Arabic numeral marks following the introduction of the British Railways Mark 1 in 1951, with further designs being referred to as the Mark 2 (introduced in 1964), the Mark 3 (introduced in 1975), and the Mark 4 (introduced in 1989). The Mark 5 designation was initially intended for coaches belonging to the 1990 InterCity 250 project, but this was cancelled in 1992; the designation was later applied to CAF-built coaches used on the Caledonian Sleeper service since 2019, replacing earlier Mark 2 and Mark 3 coaches, while coaches built by the same manufacturer for TransPennine Express are referred to as the Mark 5A.
The United States Navy uses the terms "Mark" and "Mod" as a method to uniquely designate specific types and configurations of equipment that would otherwise lack military designations. The practice was adopted by the Naval Ordnance group in 1944, and was formalized in the MIL-STD-1661 MARK and MOD Nomenclature System[9] [10] in 1978. As the system came from the Ordnance group, it is primarily used to describe naval guns, gun mounts, and other similar weapon systems.