Unit History: 9th Yorkshire Regiment

I have a 1914 English half crown engraved on one side with the following details around the inner edge:
 
T7933 CSM PEACOCK .A.   9th yorks Regt
 
and in the centre  of the coin,    WES
                                20.12.18
 
I was hoping to find some info re this and any history relting to someone who would deface a coin (in those days of some value) obviously not expecting to survive.