Round bronze pin with "For Service at the Front" and a union jack on a shield in the centre. "C.E.F" is overtop of the shield. The back of the pin has "Penalty for misuse 500 dollars or 6 months imprisonment. The badge has been altered - the central pin was removed and another pin was soldered on either side. One side of the alteration obscured the serial number that was on the pin.

The history of the badge can be found on the Veterans Affairs website:
"The Initial CEF issue is a bronze button 14/16 inches (22 mm) in diameter with a screwback fitting; the outside a circle with the words FOR SERVICE AT THE FRONT above and . . + . . below; the centre an enamel Union Jack in the form of a Tudor shield on a pebbled ground; this is surmounted by C.P.F. (Canadian Patriotic Fund); the reverse with stamped serial number. On a subsequent issue of the badge, C.P.F. was replaced with CEF (Canadian Expeditionary Force).
Final CEF Award Criteria
Members of the Canadian Expeditionary Force (CEF) who served at the front and had retired or relinquished their commissions, been honourably discharged, or returned to or retained in Canada on duty.
Members of the Imperial Forces, subject to the same conditions as members of the CEF, provided they were Canadian residents on the 4th day of August, 1914, and had returned to reside in Canada.
Ex-members of the RAF who served only in England were also eligible for the badge if they had been "actively engaged with the enemy whilst on the strength of an operational unit in Great Britain." (National Archives of Canada, RG24, Vol. 1764, File DHS 12-3, "Circular Letter No. 50, October16, 1919. Issue of Class "A" War service Badge Ex-member of the Royal Air Force who served only in England.")" (http://www.veterans.gc.ca/eng/remembrance/medals-decorations/war-service-badges)