What kind of socket is that?
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 ;)