Here we have some weird and wonderful techology of the past, mostly because technology of the future is hard to come by.
Linesman telephone TS22L, The thing I found interesting about this particular device was the fact the PCB was coated in tropicote (Nail Varnish) to keep it near waterproof over the years. To stop moisture from entering the paper speaker cone a thing polythene is used as a barrier allowing sound though. Fanstastically simple aproach to a complicated design.
The reason it brilliant is because AVO didn't entirely create it. It was made by Gossen. Originally called the Metrahit 16. It does however have great HRC fuses and brilliant quality components and track separation, designed for industrial use.