The Field Gun featured in the photograph appears to be the 15 Pdr. "Ehrhardt" gun ordered in great secrecy from Germany in 1900; once the shortcomings of then-current British artillery were exposed during the early stages of the South African campaign. According to my references, the "Ehrhardt" Field Gun did not go into service during the South African Campaign. As a matter of interest, the Ehrhardt gun had a recoil system built into the carriage; the gun did not need to be repositioned after firing. It was a very advanced type, indeed.
The photograph dates from the period after the ABW and before the Great War.