This is a German manufactured
loudspeaker driver unit that predates the moving coil variety.
It's a "moving iron"
type that was sometimes sold to home constructors' prepared to
fashion a cone and a mechanical framework. This sort of device
could also be used to power a horn speaker popular in the 1920's.
The impedance of the coil forming the activating inductor was
sufficient to allow it to be placed directly in the anode circuit
of the audio output valve and this example has a number of different
tappings allowing best matching and consequently best volume.
Basically there's a small iron-cored
inductor carring a core with an incomplete magnetic circuit.
In the air gap is placed an armature, precisely positioned, by
means of the adjusting arrangement, to the centre of the gap.
A lever connects the armature to a tensioning device to which
is fastened the loudspeaker cone activator.
The simplicity of the modern
moving coil speaker seems to make one wonder why the moving iron
types were ever invented.