This is a Mechanism Ltd Model M1991/A mechanical digital barometer.  It was the predecessor to the Negretti & Zambra M2236 barometer, N&Z took over Mechanism Ltd in the 1970s.  These were made in the UK in the 1960s, first produced in 1962. 

​It is an aneroid barometer with an odometer-type mechanical digital display.  To operate the barometer, the toggle switch is held to the right and the knob on the right side is rotated until the green light in the window on top of the barometer changes.  When the light change occurs, the digital display shows the pressure.  It has a range of 900 to 1050 mb, with display to the tenth of a mb. It is used while mounted in the oak case.  It uses one 1.5 volt C-cell battery for power, which is replaced by removing the 4 screws holding the front plate.  I found this one in Australia, it still seems to work.

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