Rev Counter                  

The original Rev Counter was an 80mm Jaeger chronometric instrument, reading to 6000rpm.  (The 80mm refers to the diameter of the case, or the hole it fits into.)  There are various Jaegers that look vaguely similar but are incorrect, in particular the ones with magnetic mechanisms, which have the cable drive in the centre of the rear case.   Chronometrics have the drive off-centre. 

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Chassis 7231

Chassis 7308