The present lot has the case numbers and RWC LTD (the mark of Rolex UK) stamped in the case back this was done for at the request of the British Assay Office in order to ensure that once a watch was imported into the U.K. and the duty paid the back could not be swapped to another watch to avoid import duty The above fact and the fact that this watch still retains its original Rolex sticker on the case back is excellent proof that watches left the Rolex factory with double references. The present lot bears two reference numbers 1611 between the lugs which refers to the specific characteristics of this watch and Ref. 1601 in the case back. References 1611 and 1601 are identical with the exception of the bezel. Calibre 1575 is an improved version of the calibre 1570. The bridge of the calibre 1575 is however still engraved 1570. This watch is illustrated in ?Rolex Collecting Wristwatches? by Osvaldo Patrizzi 2001 Edition p. 245.