Here's the BC221 with its front
Batteries are held inside the bottom
of the case.
The equipment consists of a stable variable
frequency oscillator having a vernier dial, a crystal oscillator
and some circuitry, which after twiddling the appropriate knobs,
enable a squeak to be heard in headphones when coupled to a transmitter.
By comparing the dial reading of the squeak with a set of tables
carried in the lid a pretty exact frequency reading of the transmitter
output may be obtained.
Alternatively one could set the dial
to a precise setting and tune a transmitter or oscillator, to
which the BC221 is connected, to a pre-determined frequency.
The set of cards in the lid were very
important as without them the wavemeter would be useless. To
keep the cards in place they're held in a booklet with metal
covers screwed to the drop down door.