You appear to have a Mk IVA standard revolution indicator for aeroplanes, that someone has modified to read to 2200 RPM, from the required calibration range of 800 to 2000 RPM.
Royal Aircraft Factory specification No. 70., dated 10 Nov 1914 (with my copy printed 17 March 1917) covers this instrument. Details are on drawing No. 25 Z.A.
The Mk V specification (RAF 130) is dated 23 Jan 1917 required a calibration range of 600 to 2600 RPM.
The Mk IVA specification indicates that it is for the use with the following engines:
70 H.P. Renault
100 H.P. Green
No. 1 R.A.F.
80 H.P. Gnome
100 H.P. Gnome Monosoupape
120 H.P. Austro-Daimler
Details are given as to what gearboxes etc. are required to be fitted for which engine.
I had not noticed before but the dial is required to be as follows:
17. The dial is to be silvered and to be the same shade as sample supplied by S.R.A.F.
18. All engraving to be in black.
19. The flange and case to be of dull lacquered aluminium.
Basically saying a white dial where as for the Mk V:
20. The dial is to be dull black with white figuring and lettering.
21. The pointer is to be white with the balance part black.
22. The flange and screws are to be painted and stoved dull black.
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