This 3cm diameter identity tag is made from a German East Africa One Rupie coin. It seems to have been popular with Commonwealth troops serving in the East Africa campaign to use German 1 Rupie coins as identity discs by polishing off the Kaiser’s image and stamping in the man’s name and unit to the front instead. This identity tag belonged to C. E. Martin of the 2nd Brigade, 7th South African Infantry Regiment (2.B. 7 S.A.I.) This unit was severely mauled at the Battle of Salaita Hill on 12 February 1916 in German East Africa 

http://samilitaryhistory.org/vol166as.html

See also: 

https://en.wikipedia.org/wiki/Battle_of_Salaita_Hill

A great piece of history from a famous unit engaged in a famous battle