T40 plug at bottom of gearbox- what for?

I replaced my gearbox oil over the weekend on my 1992 Mk1 cabby. Initially I removed the T40 bolt at bottom of gearbox thinking that it is the drain plug. To my disbelief only a drop or two of fluid came out before I realised the drain plug is the 17mm hex plug further back on the gearbox. Luckily about 1.8L of oil came out so the level was not very low. It was dirty so worth changing. My question though is what the other plug is for and should there have been any oil in it?

Thanks

 If its the bolt I think you describe it locates and locks the reverse gear shaft in place.
You wouldn't have done any harm removing it and the oil would just be a small amount that's gathered between the bolt and the shaft as the shaft hole is blind.