The Maritime Rescue Coordination Centre has revealed that five containers fell overboard a Chinese ship, Anguan Gjing, en route from Durban to Nigeria in recent heavy seas..
The authorities say the containers are empty and pose no risk.
Three containers washed up near the Gamtoos River mouth while a fourth was spotted off St Francis.
Authorities have yet to decide on what to do with the containers.
See previous, related post about the first beached container:
St Francis Chronicle 
So, not a sinister canister then.
Googling Gamtoos to find out more news about the object on the beach, I found this interesting information about HMSAS GAMTOOS and it’s service during the Second World War, http://samilitaryhistory.org/vol135vw.html
It makes interesting reading.