The present watch is part of a small series of Mark XI models made without case numbers or military markings. It is accompanied by the photocopy of IWC letter dated 26 February 2003 confirming that a limited number of Mark XI models with "plain" screw backs were produced in the early 1960s. The backs of these watches are neither numbered nor featuring IWC's signet nor military markings. However they bear IWC's signature as well as the casemaker's, FH.
Production of the Mark XI, one of the most legendary pilot's watches, was launched in 1948. The model was always fitted with the IWC calibre 89 movement, with or without the hacking feature, and cased in stainless steel, some early versions in chrome-plated brass.
It was issued to the British Ministry of Defence and used by the RAF as well as Commonwealth forces, including South Africa, New Zealand and Australia. The engraved numbers on the back differ depending on the service. Dials and military markings also vary but those for military use show a broad arrow, some, as the present watch, also the Ministry of Defence specification for tritium, the encircled T.