The tributes from the Stornoway Gazette for 1918 have yielded another tidbit to be able to add more information to one casualty: Roderick Macdonald, 23 East Street, Sandwick. His date of death was marked as unknown in the Roll of Honour. However, a date of death of 11 April 1918 from the tribute has allowed me to track the following CWGC information.
Lance Corporal RODERICK MACDONALD
Last address in Lewis: 23 East Street, Sandwick
Service unit: 2nd South African Infantry
Service number: 13116
Date of death: 11 April 1918
Killed in action in France
Interred: Messines Ridge British Cemetery, grave V. C. 14
Local memorial: Lewis War Memorial
There was a South African contingent serving on the Western Front, but I have (as yet) not been able to track Roderick down on their on-line files.