Mark I or Mark 1 often refers to the first version of a weapon or military vehicle, and is sometimes used in a similar fashion in civilian product development. In some instances, the Arabic numeral "1" is substituted for the Roman numeral "I". "Mark", meaning "model" or "variant", can itself be abbreviated "Mk." Military and weaponry Mark I tank (1916), the first tank to be used in combat Supermarine Spitfire Mk I (1938), Royal Air Force fighter aircraft Mark I Fire Control Computer, United States Navy fire control computer used in World War II surface ships Mark I Nuclear Weapon (Little Boy), first nuclear weapon used in combat The Mark I NAAK, an auto-injector carried by military personnel for use in case of nerve agent attacks Mk 1 Underwater Defense Gun, 1970s United States Navy dart-shooting underwater firearm Mk 1 grenade, the first American-designed grenade used by American forces in World War I Mark I trench knife, a combat knife carried by US forces after World War I Patrol Boat, River ("Mark I PBR", 1966), 31-foot version of the US Navy riverine patrol boat Vehicles Mk I Mini (1959-1967); the original Austin Mini and Morris Mini-Minor from British Motor Corporation British Railways Mark 1, the first standardised passenger-rated rolling stock (carriages or cars), introduced on British Railways in the 1950s UTDC ICTS Mark I, rolling stock used by Vancouver SkyTrain rapid transit Other technologies MARK 1 or Perceptron (1959-1960), a neural net computer designed by Frank Rosenblatt at Cornell University Mark I (detector), a particle detector at Stanford Linear Accelerator Center from 1973 to 1977 Colossus Mark I (1944), a British computer used to crack military codes Ferranti Mark 1 (1951), an early computer based on the Manchester Mark 1 GE BWR Mark I boiling water reactor, a Generation II nuclear reactor Harvard Mark I (1944), an early automatic digital computer made by IBM The Lovell Telescope, called the Mark I between 1961 and 1970, then the Mark IA between 1971 and 1987 Manchester Mark 1 (1949), an early Autocode computer Mesa Boogie Mark I (1969), an electric guitar amplifier designed in Northern California Other uses Mark 1 or Mark I, the first chapter of the Gospel of Mark in the New Testament of the Christian Bible Visual inspection, sometimes been called Mark I Eyeball in the US Military since the 1950s Patriarch Mark I, retronym for Mark the Apostle