The Miller is not intended to seal there. The intention was that the prmary case can breath through the gap and if you seal it with silicone then it can cause condensation inside the primary case area. ET164 is pressed into the crankcase to act as a top hat so any oil running down the back area is diverted away and cannot reach the hole on the crankcase. PD28 is an oil thrower is so any airborne oil is spun off before it reaches the open gap. The important bit is the PD28 oil thrower and ET164 top hat should have minimum running clearance to work and if the gap between them is too large then oil gets out and its not unusual for owners to remove ET164 top hat from the crankcase because it was catching the oil thrower.
I have 3 twins on the road and one I have done over 50,000 miles and all have this gap and none of them leak oil at the dynamo area.
When I first got the Rapide on the road 30 years ago as a novice I followed the advice of the time and initially sealed the dynamo to the cases with sealant and got condensation and milky oil in the primary chaincase which is not good for primary chain life. I removed the sealant and set the dynamo drive up correctly and never had a problem since. I will admit all my bikes now have a proper oil seal on the driveside mainshaft so the engine doesn't breath into the primary case which may help.
I have 3 twins on the road and one I have done over 50,000 miles and all have this gap and none of them leak oil at the dynamo area.
When I first got the Rapide on the road 30 years ago as a novice I followed the advice of the time and initially sealed the dynamo to the cases with sealant and got condensation and milky oil in the primary chaincase which is not good for primary chain life. I removed the sealant and set the dynamo drive up correctly and never had a problem since. I will admit all my bikes now have a proper oil seal on the driveside mainshaft so the engine doesn't breath into the primary case which may help.