Thank you, karesz* - a very interesting discussion! I was particularly interested by the first picture attached, which shows the interior of a Greiner microphone unit, and here's the link to the
RS vibration sensor the poster thought was similar.
I happen to have one of the same models of microphone, and on taking it apart (second picture) see it's suffered the same fate as the one I opened this thread with: the encapsulation of the sensor has completely disintegrated. Interestingly, the exposed contact is the same shape as the RS part, but obviously used to be part of a larger square unit glued to the bar that passes through the casing to become the clip that holds the movement.
I've received some standard small disc piezo transducers which I will try. Does anyone have any soldering tips for these, apart from very flexible wire? One to the brassy disc and one to the smaller silver one, with one side bonded to the clip?