See:
http://www.pelicanparts.com/911/911_Parts/1974-75/3-2.JPG
http://www.pelicanparts.com/911/911_Parts/1974-75/3-2-1.JPG
Part 14 in the first figure is the vent fitting and it's on the top.
I suppose given the thickness of the transmission oil (especially if it's cold) and if the transmission wasn't level, one could put too much oil into the transmission by pouring in through the vent hole and waiting for it to come out of the actual fill hole (part 11 in the second figure). However, if the vent fitting hole was used to fill the transmission it is more likely the fitting was re-installed with the wrong orientation. This allows the tube hanging down from the vent fitting to act like a scoop and collect oil thrown up by the differential gear and direct it out the vent hole. Gravity and the air flow under the car takes over from there to spread Vulcan blood (Romulan ale?) everywhere. Cheers, Jim