Steel ball on the rear side of engine block,what is it for ???

Hi everyone,mine is Pajero io 1999 1.8,petrol 5 door MPI,SOHC.I had to take gearbox  out for rpairs and also to check small oil leak coming from rear of the engine.Took pressure plate and clutch plate out then flywheel.Upon inspection realised that rear main seal is still ok it was dry and no sign of oil leak.Also took our rear plate and descovered this steel ball. (Check the picture)It is probably 12-14 mm in diameter.I understand roll of welch plug (freeze plug) but not sure about this steel ball. Any suggestion, ideas please share...It seems that oil was seeping from top of engine block...??

Kind Regards    Voya