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.
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.