A massive metal box and I bought it out of curiosity and because it was a local pick up. It turns out it was used as a railway telephone.
It had a unique Western Electric selector, which allowed selective ringing of a specific telephone on a single line, with several of these telephone connected at different points. Kind of like an advanced party line system.
The thing weighed a tonne and was made in 1952 (the date on the capacitor). It was used by the NS (Dutch Railways) and the card on the front shows it was used in the Zwolle district.
The card reads:
CENTRALE SCHAKELPOST ZWOLLE
A. Luisteren, of de lijn vrij is
B. Melden met naam en plaats
C. Wachten tot CSP zich meldt
Indien de lijn niet vrij is, telefoon ophangen en later opnieuw luisteren
OPROEP VOOR POST
A. Bel gaat over, klepje valt
B. Telefoon opnemen en melden
Which translates as:
CENTRAL SWITCHING STATION ZWOLLE
Call CSP (Central Switching Station)
A. Listen for a clear line
B. Call name and place
C. Wait until CSP answers
If the line is not clear, hang up the phone and listen again later
CALL FOR STATION
A. Bell rings, drop flap indicator falls
B. Pick up telephone and call in