Does yours have it and is it open?
Does yours have it and is it open?My first instinct was a port for a redundant coolant or oil line. That or for a hydraulic clutch line. The slave cylinder usually goes on the left side of the engine but could be where it goes into the gearbox.
Looking at Google though, the 821 and 1200 motors both have the same thing. Some kind of breather maybe?