This is the Rapid Bandchecker, an absorption wavemeter for checking the multiplier stages in a transmitter to ensure they are tuned to the correct harmonics. As far as I’m aware, this hand-held device was manufactured by Raymart of Birmingham, UK in the mid 1950s.
Range 1 covers 3.5 and 7 MHz.
Range 2 covers 14 MHz.
Range 3 covers 21 and 28 MHz.
In the good old days back in the 1960s, we had to use a wavemeter to check our amateur radio gear. The GPO sometimes visited radio amateurs to check that they owned a decent wavemeter. I believe the popular one was a Class D Mk 1 wavemeter. As I recall, one chap had a Class D wavemeter and it would do the rounds if the GPO were in the area.