I've recently written about the Natal Police (NP), a unit previously known as the Natal Mounted Police, in my blog at
http://molegenealogy.blogspot.com/search?q=natal+policeThere are a couple of links included on that page which may be of interest.
The pic (uniformed group) would be a bisley group of NP, and as indicated by the date 1912 on the cup the photo would have been taken quite late in the NP's career - they ceased to exist in 1913.
It would be very difficult to identify the men in these photos, unless by some miracle the same pictures were preserved in, say, Pietermaritzburg Archives Repository, with all the names given. A photo of a 'special' group made up of members of a unit (NP in this case) to perform some specific function would be unlikely to be preserved other than by the people who appear in the photo.
You may already know there is a deceased estate file for your Robert Irving Johnston held in Pietermaritzburg:
DEPOT NAB
SOURCE MSCE
TYPE LEER
VOLUME_NO 0
SYSTEM 01
REFERENCE 20273/1933
PART 1
DESCRIPTION JOHNSTON, ROBERT IRVING. (S/S MARGARET ALICE; BORN BARNES).
STARTING 1933
ENDING 1934
There are service records of NP - see my blog for more info.
Re 'Nongqai': they were the Zululand Native Police established in 1883 as a bodyguard to Sir Melmoth Osborn, Resident Commissioner of Zululand. This respected force was known as the Nongqai; their fort at Eshowe is now a museum. The Trophy mentioned would have been named in their honour.
Mole