I am attaching a scan of a postcard bearing the stamp of FPO No 25, which I believe was in use at Kajaido Railhead at the time. The card is of a group of native warriors and the words "Deutsch-Ost-Afrika"
Sgt Arthur Cowbourne was with the 11th (Heavy) Battery, Royal Garrison Artillery in East Africa in 1917. He was awarded the DCM for gallantry, gazetted 3rd October 1918, but I have not found where this action took place. His battery was apparently later on the Western Front but the citation speaks of "the forward elements of the column", which might suggest it was while he was in Africa.
Can anyone shed any further light on this?