Miller horn internal wiring query

Can anybody help me diagnose what's wrong with the Miller horn on my Norman Nippy? When I press the horn button I get a short 'bip', but not a continuous tone. The electromagnetic coil is working fine and attracts the metal disc down onto the electromagnet, and the disc simultaneously touches the two big metal contacts that sit outside the coil. Presumably, when the disc touches the two big metal contacts it is supposed to earth the flow of electricity, thereby causing the magnetism to collapse and the metal disc springs away; only it doesn't earth it. In fact, there is no measurable voltage in either the core of the electromagnet nor the two big contacts to the side of the coil, so there's no flow of electricity to be earthed, therefore no switching mechanism. I'm now very confused as to what's broken - could be anything after 40 years in a leaky shed preceded by years of abuse! Two photos are attached of the inside of the horn.

Anyone got any idea what's wrong here? Thanks in advance for any thoughts.
Charles Fox