WWI "On War Service" badges

[Spuffler]

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)