You have a magnetic wire in your hands
The magnetic wire is an analogue audio format. The magnetic recording is located on a thin steel wire of about 0.004 inch (0,1 mm) thick, which is wound around a reel.
… that there is also a miniature version: the minifon? This small magnetic wire has an audio capacity of 2 to 5 hours. Because of its compact dimensions, it is ideal for making secret recordings. There even exists an ‘invisible’ microphone, shaped like a watch.