Mark I was EOC Pattern "Z" and was of built up construction while Mark II was a Woolwich design.  The Mark III was known to EOC as "Z¹."  All used a 3-motion breech mechanism but most were later converted to a single motion breech mechanism.  Guns so converted had a "B" added after the Mark number.  A total of 137 Mark I, 760 Mark II and 53 Mark III guns were made.  Some of these guns were employed as AA weapons during World War I, but this did not prove very successful.  63 Mark II guns were transferred to the Army, where they were bored out and converted to 8" (20.3 cm) howitzers.

Nomenclature note:  The 6"/45 (15.2 cm) QF Mark IV designation was given to twelve guns purchased from the USA firm of Bethlehem Steel early in World War I.  Eight of these were used in coastal defenses at Scapa Flow and four were used on DAMS.  These guns may actually have been the Bethlehem 6"/44 (15.2 cm) Mark 9 or a close relation.

HMS Furious in 1898
6"/40 (15.2 cm) gun in bow pedestal mount
U.S. Naval Historical Center Photograph # NH 61062

6"/40 (15.2 cm) guns on HMS Hermes
The original postcard is titled "Captain Scott's Dotter" which was a mechanism used to teach gunsighting techniques
Photograph courtesy of Brian Cave and Paul Benyon

Note the three motion breech and sailors holding rammer, projectile and propellant casings
Bain News Service Photograph
Library of Congress Photograph ID LC-DIG-ggbain-16785

See photograph 6487