I have a few problems with this one
The King's Own Royal Border Regiment (KOSB) did not take part in the Zulu War, they were fighting in Afghanistan (1878-80).
They were in South Africa in (1840-42).
Having no dates to work with, I am unable to tell if he is talking about 1840-42 or 1879.
Bell is a common surname, so without a first name, having trouble locating him.
John( Bell) is the author of the work and does not mention his Grandfathers first name.
I can find a George Bell, Drummer, 94th Regiment taking part in the Zulu War and a James, William and Thomas Bell's listed as Privates also taking part in the Zulu War in the 94th.
I do think a little more information is needed to close the case of this one.