I found that socket on a music device produced around mid 1970s in GDR and I was wondering what kind of socket it is. Unfortunately, someone painted the whole device, including the contacts. Therefore, I plan to replace the socket with an IEC-320-C14.
Additional Information
The device is a so-called "Kofferharmonium" (kind of a very big harmonica) manufactured by "VEB Klingenthaler Harmonikawerke". This page has some more information on it. The third picture shows the electrical internals of the device. My machine has no switch or regulator, but the motor is directly connected to the socket.
There is no schematic or any other hint on / inside the device.
Best Answer
This is KN-4A socket. Widely used in Eastern bloc countries in many types of electric equipment with a motor, mostly in sewing machines.
What is important - this socket is not provided for powering up motor or a whole device directly from mains. To make a full circuit you need a steering pedal and special cord. Pedal and cord looks like this (note similar plug but with two contacts):
Pedal controls the speed of the motor, inside of pedal there are switch, capacitor and potentiometer. Or sometimes a rheostat. Or sometimes even a couple of switches and resistors - the more You push the pedal, more resistors are just shorted and motor speed increase. Pedal is connected in series with one of the motor windings. Motor is single phase induction type.
Buying sockets and cords is not a problem, they're still in production because many people still use these (not so) old but very good sewing machines. The best solution - find the original pedal. Or find the parameters of original pedal and choose a similar sewing machine pedal.
If You omit the pedal the motor will be running constantly at full speed. Firstly, it doesn't like it. Secondly - sound from Your harmonica will be awful ;)