hosting immaginiThe "Mark XI" was really the "Mark 11". The British armed forces "Mark" numbering system went to Arabic numerals after the end of WWII, and the Mark-system was discontinued after the Mark XI. The Australians stuck with the Roman numerals, though. The Mark XI is also known as the 6B/346.
The "Mark XI" was originally purchased from both IWC and JLC, but the RAF was dissatisfied with the JLC version (no shock protection) and discontinued buying them, leaving IWC as the sole RAF supplier. Ca. 2'000 watches were procured from JLC in 1949, and ca. 7'400 from IWC between 1949 and 1953. All JLCs were decommissioned before 1963, and the IWC watches in 1981. In 1973, the Mark XI was downgraded to "navigator's watch secondary type".The specification for the Mark XI stipulated chronometer accuracy. The watches were regulated in five positions and at least two temperatures over a two-week period at the Chronometer Workshop of the Royal Greenwich Observatory in Herstmonceux, and kept in sealed storage for 12, later 18 months, and then re-regulated, whether they had been used on the flightline or not. The precision expected of the Mark XI and the care taken in its maintenance make the Mark XI a much closer relative to the German B-Uhren of WWII than the mass-produced American A-11, A-17, DTU 2A/P and GG-W-113 watches of significantly lower accuracy.
Only aircrew navigators were originally issued the Mark XI (another parallel to the German B-Uhren), and if there were enough to go around, first pilots and captains could be issued them as well. Other aircrew, like flight engineers, received the lower-spec'ed Omega 53s and older Omegas.
The Mark XI was specifically designed for astronavigation which had become a feasible option only with the wide-spread introduction of the "bubble sextant" in the early 1940s.
The requirement for anti-magnetic shielding (and anti-magnetic hands, by the way) stemmed from the use of an airborne terrain-mapping radar (H2S) in the final phase of the war. Its huge magnetic field output negatively affected the aviation watches (Mk. VII=6B/159 and Mk. VIII=6B/234) then in use. Likewise the requirement to secure the crystal against the loss of outside pressure came about with the introduction of cabin pressurization towards the end of WWII when older watches "popped" crystals during rapid depressurization (e.g. when the cabin was hit by a shell fragment). Increased altitude, and therefore increasingly cold temperatures, affected the accuracy of the Mk. VII and VIII watches. A steel case was required as the chrome-plated brass or alloy cases then in use did not fare well in use over salt water and in humid climates.
The bonklip bracelet (6B/2763) was originally standard issue on the Mark XI. It was discontinued in favor of the standard nylon NATO strap (6B/2617) in 1954, but re-introduced in 1956 as an alternative to the NATO. There are reports on Mark XI on long leather straps, but they are uncorroborated. Either way, the standard NATO strap (I don't know which color, though) is quite a correct period strap for the Mark XI.
Helpful Links:
http://www.fortunecity.com/olympia/w...79/Mark.XI.htmhttp://www.markeleven.com/
