[Arran]

A home brew set would be my guess as well, both because of having no markings and because it only has two dials. Building a two dial set is simpler then a three dialer and requires fewer parts as there is at least one less tuned circuit. Some inside shots would be helpful, but I do like that marbling in the front panel.
Make sure that you bring a 9 volt battery along to test the driver on the horn, if it's good you should hear a click, if not then either the leads have a problem or the coil in the driver is shot.
Regards
Arran