This is an old On War Service brass and enamel badge from 1915. It was made by JR Gaunt London and has a number 11578 on the reverse.
It has obviously been worn and has enamel damage as can be seen in the images. Also some tarnishing to the metal on the back side. It is over a hundred years old though so it’s a nice honest and used example. Belonged to someone who helped with the war effort at home.