This is an NGG11 of either NG51/52/53, the first order which had slide valves, you can see this on the right front cylinder. NG51 did go to Natal at sometime, so maybe if this is the loco the mechanical lubricator was fitted as an experiment or it could be one of the other two fitted after the fact.

I could not find any definite info on this however as all the 1st order NGG11 pics I looked at don't show the lubricator as fitted in this instance.