AP1275 1930 “General Instrument Equipment for Aircraft” Sect 1, Chap 1 :

“Mark VI Revolution Indicator
As the dimensions of aircraft increased, it was found that standard types of revolution indicated(sic) needed exceptionally long flexible drives, more particularly in the case of multi engined aeroplanes whose engines were carried in nacelles on the wings at some distance from the pilots cockpit..trouble was experienced when the flexible drive exceeded 30 ft..there arose a demand for dials for the use of the engineer..led accordingly to the development of a large dial indicator.

The MkVI has a dial of 5 inch diameter, graduated 600 – 2600 RPM, movable lubber mark, weight 3lbs 12 ozs.”

3500 RPM is associated with V12 engines, contemporary Hawker Hinds with Kestrels had 3000 – 3400 RPM Mk IX indicators, which AP 1275 1937 shows came after the Mk VI. As aircraftclocks has pointed out, perhaps this was a Mk VI special order.

If the guts of the dial stayed the same as a 2600 RPM unit and only the dial face changed, you would only have to alter the engine output ratio for the needle to traverse approximately 300 degrees around the dial, which is a product of the engine gearing ratio for output to the flexible drive. So what apart from the V12s output this ratio to the RPM indicator ? Armstrong Siddeley Serval ? Saro Cloud flying boat ?
I wonder if this is an early thirties gauge from multi engined type using RR Kestrel, HP Heyford, Short Singapore, Supermarine Southhampton ?

Nice gauge.:)