FWIW, there is a drain plug at the bottom of that primary cover that allows you to empty the mist collecting from the primary chain lube & clutch. If you forget about it for 15 years, the level eventually builds up to the point where it weeps past the paper gasket on the Derby cover when the bike is parked on the jiffystand, runs down the case and the dog marks it's spot.
All of which does not explain why sometimes they leak in the same spot when *new*.
The Evo engine improved it quite a lot, but even to this day, you’ll get the occasional new Evo-engined bike that marks its spot - something that the rest of the industry considers unacceptable. Even Triumphs don’t leak any more.