To fit American or Canadian 500 Series telephone.
This is a superb quality Electronic microphone giving modern speech quality shown on the left in the pictures. Internally the electronics are sealed to prevent the damp from breath effecting the performance. This is very easy to fit and simply requires undoing the transmitter cap and removing the old carbon model and fitting the new one and screwing the cap back on. This should take less than a minute to fit. Note I have taken pictures of the electronic model next to the old carbon model to show it is the same size etc. You are buying the model on the left side only.