A Nato attack frogmans compass as used by Royal Navy Clearance divers and Special Boat Servive (SBS).  Military issue brass case with liquid filled compass and leather strap. Stamped to the case with 'PATT 261' and 'No 696B/55' and dating to around the 1950's.  Diameter 6.5cm.
Sometimes these were fitted with a lanyard which divers hung off their equipment or also fitted to a swim board  worn around the neck with a depth gauge and the highly collectable Rolex watch.