Do you mean the M10x1 hole right round the back? If so this is tapped into an oil way, used on the MK2 for the MFA oil temp sensor.
If you mean on the side near the coolant flange, 2 of the M10x1 holes on the side are in coolant channels, the other M10x1 hole isn't tapped into anything, its a blind hole. The last M14x1.5 hole is tapped into a coolant chanel, on the MK2 its used for the 5th injector thermoswitch.