A soldier became eligible for a campaign medal because he served in a particular war within a specific date parameter. No medal was struck especially for the Anglo-Zulu War 1879. Instead, a medal previously awarded for campaigns fought on the Eastern Cape frontier during the 1850s was, with some slight changes to the design, awarded to all those who served in South African campaigns between September 1877 and December 1879 - including the Zulu War.
A bar or clasp showed the year or years in which the recipient served in any of the 1870s campaigns. The bar '1879' worn with the medal indicated that the recipient had served in Zululand during the Zulu War, while men who had served in Natal but not crossed the border into Zululand received the medal without the bar.
Together with the information concerning the recipient which is inscribed on the rim of the medal, the presence or absence of the clasp is significant.
It was suggested that a special clasp be provided for those who took part in the defence of Rorke's Drift but this was not acted upon.
At the time of the Zulu War the Victoria Cross and the Distinguished Conduct Medal were the only awards available for valour and for gallantry in the field. Eleven Rorke's Drift defenders received the Victoria Cross and five other defenders the Distinguished Conduct Medal. The total number of V.C.s awarded for the Zulu campaign was 18. A list of all recipients of decorations for the campaign can be seen at http://samilitaryhistory.org/vol044fm.html
Medal Rolls are at TNA under WO 100. Check the 1877-9 South Africa Medal Roll to find out if your ancestor was a recipient of this campaign medal.
The South Africa Medal Roll 1877-1879 for the Naval Brigade who served in the Basuto and Zulu Wars can be found under ADM 171/40 at TNA, Kew.