Omega. A stainless steel automatic diver's calendar wristwatch Seamaster 600 'PloProf', Circa 1970Omega. A stainless steel automatic diver's calendar wristwatchModel: Seamaster 600 'PloProf'Date: Circa 1970Movement: 20-jewel automaticDial: Blue, luminous baton hour markers, white outer minute divisions with luminous five minute markers, date aperture at 3, white and orange sword hands with luminous inserts, centre secondsCase: Large monocoque water-resistant form case, shouldered screw down crown with patented locking nut, bi-directional black bezel calibrated to 60 units and locked/unlocked by depressing red button in the band at 2Strap/Bracelet: Blue Omega rubberBuckle/Clasp: Signed steel buckleSigned: Case, dial & movementSize: 46mm Accompaniments: Omega service case, service receipt dated 2017, parts replaced during service This model, the Omega Professional Seamaster 600, was originally designed for use by deep-sea divers and became known as the 'PloProf', for 'Plongeur Professionel'.Though most watches are made with the crown at 3 o'clock, the standard PloProf has its crown at 9 o'clock so that the red pusher is more accessible and the relatively cumbersome crown guard does not interfere with wrist movement.When diving, one can calculate how much oxygen remains in the tank and adjust the bezel for accurate timing. By pushing the red button, the locked bezel releases and the bezel can then be adjusted.