Elliott Vibrating reed frequency meter 26 to 30 Hz 30 Volts I assume because of the scale marking AP 58134 that it was made to an Air Ministry specification. Comprising a set of tuned reeds and solenoid. For theory of operation see "Electrical measurements and Measuring Instruments" by EW Golding, Pitman 1935 page 771. Elliott Brothers (London) Ltd. were instrument makers active 1905-1967 now part of BAE Systems. This instrument probably dates from about 1943.
This is one of many instruments collected by Jack Davidson C Eng FIEE which I have been given by Dr Graham Winbolt
Interior of the instrument