Thank you for your reply,
no, it is not a pin at the back of the tube.
The tube is undamaged.
It is a hole in the glass at the side of the tube wherein a pin, attached to a wire from the horizontal amplifier, fits.
That is the pin that is broken. (see second picture of the good one and the broken off on)
The hole has a diameter less than a millimetre, (see other picture, scale is in millimetre).
Pascal