Haken,
You may have the black plastic version https://www.moto-lita.co.uk/accessories/black-plastic-horn-button/

Or the alloy one https://www.moto-lita.co.uk/accessories/morgan-engraved-billet-horn-button/

Both remove by easing your finger nails or a credit card under the outer edge and teasing it upwards, when fully out, if you unplug the spade end connector from the back and just touch to the boss metalwork the horn should work as a test

Jon M