Found this quite by chance at the SAMHM last week, from a magazine article by Lt. W.M Bisset, SAN and titled "South Africa's First Anti-Aircraft Guns":
Two 15 pounder Armstrong BLC's (breech loading cannons) were converted for use in an anti-aircraft role during the early days of WW1 by Captain C.L. Gransden and his team of armament artificers at Fort Knokke, Cape Town. The main problems which had to be surmounted was the designing of a new carriage capable of elevation far beyond the standard 16° and of withstanding the recoil because the gun alone weighed around 900 pounds (457 kilogram) and its normal recoil through its field cradle was 3' 6" (1060 mm). The first gun converted had a maximum elevation of 60° and the second had it improved to 71°.
"Skinny Liz", as this peculiar piece was named, accompanied the 1st Infantry Brigade to Walvis Bay in December 1914 for the invasion of German South West Africa and was initially mounted on the foc'sle of the S.S. GAIKA (Union Line, 6 300 tons) ready for use "should an enemy aeroplane appear". The only record of shots fired in anger at an enemy aircraft is contained in an official report on 2 Battery, South African Mounted Rifles which describes an unsuccessful engagement with a troublesome German "Taube" monoplane. The report adds that one of the recoil springs broke each time a round was fired. See also:
"Skinny Liz" was employed in its dual role throughout the German South West Campaign "with considerable success" (I.V. Hogg and L.V. Thurston: British Artillery Weapons and Ammunition 1914-1915, pg. 74) but it would seem that its weight and lack of success in its anti-aircraft role may have led to the decision not to use the conversion in another theatre of war.