WWI "On War Service" badges

Does anyone know anything about the "On War Service" badges issued to people in reserved occupations in WWI? My father had one when he was an apprentice in 1915. A friend brought his collection of them to show me yesterday, and none were anything like the one my father had. The friend thought they may have been issued by factories; perhaps someone has some information on them, and could comment on this assertion?

Are there any relating to Coventry or Coventry firms known of? (My father's was from Birmingham - he worked for a firm of ship lamp makers)