Whilst continuing to get to know the WNG I found the word DRAIN stamped into the outer primary case just above one of the case securing screw holes. Inside at this point the gasket face is relieved so that when that screw is removed the oil can run out of the screw hole. Per the parts lists for WNG and civilian singles the part numbers, and casting numbers, are the same but clearly the WNG ones were modified to allow the oil to be changed without having to remove the clutch and separate the cases.
If you look closely at the relieved area you can see the casting marks so it does not look like it was a case of a chap grinding that area away to modify a standard case but that the case mould was actually altered to produce the relieved area.
However, in my mind it looks like a prime place for a leak if the screw does not fully seal against the countersunk area in the case hole.