The word mark, followed by number, is a method of designating a version of a product. The kind of products that use this convention vary widely in complexity. The concept shares some similarities with both the "Type (designation)" (Hardware) and the 1.0+ (1.1, 1.12, 2.0, 3.0, etc.) Software versioning convention often used to designate general software product releases. It is often abbreviated as Mk or M. Because a mark is often made to measure height or progress, by metonymy the word mark is used to note a defined level of development thus designations like "Mark I", "Mark II", "Mark III", "Mark IV", etc. come to be used as proper names. However, since the same name is used for a wide variety of products, it can have varied connotations for different persons.
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Usage
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.
United Kingdom
In British military practice, Mark designations have been given in Roman numerals, often as sub-designations of "Number" designations. For example, the Number 1 rifle, the Number 2 rifle, etc. were major types of equipment, with "Mark" designating a minor variant or production change, i.e. "Number 1 Mark III" or "Number 4 Mark I." However, British Railway practice has usually designated things using Arabic numerals. For example, coaches produced by British Railways went through Mk1, Mk2 and Mk3 series.
United States Navy
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 MIL-STD-1661 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.