The only joint above the filler plug is between the main casing and the top cover, which doesn't have a gasket and I've never known leak, plus the plate that holds the detent springs in place, which can weep but I've not known pour oil out of there either, as there isn't much of an oil supply there. Obviously there is a gasket between the main casing and bellhousing, but I can't see that getting to where you say the oil is coming from. There are also selector shaft seals, but they tend to run off down the back of the box, not down the side. I reckon you need to get under there with a torch and the Mk1 Eyeball.