Whilst rebuilding my gearbox I noticed that there is only a 1/4" hole between the outer case and the inner chamber to allow oil to level between the two. Whilst this is probably adequate, a further problem is that that hole is half covered by the gasket. I cleared it out with a drill turned by hand but it would be more satisfactory to trim the gasket before assembly.
The outer gasket is quite broad and this serves to ensure that debris that is washed down into the gearbox when you lube the cable, runs down the inner face of the outer cover and hopefully stays outboard of the gasket, avoiding contaminating the main blob of oil. I was surprised at what was sat in the outer cover.