Do I have a real oddity here? I just went out to the garage again to make sure I wasn't imagining this. My bike is an '83 R65. At the bottom of the transmission housing where it meets the engine case is a gap of approx 2x16cm. It is open to the clutch. I assumed that is must be some design for ventilation? I don't know. It is placed where it would not be possible to take water unless the bike was submerged. When I got my bike it was expecially dirty and black dusty here and I sprayed it well with a pressure washer. Some time later my output splines disintegrated on me and I (thought anyway) realized that the dust must have been from the clutch and I'd sprayed up into it without realizing the clutch was exposed.
I just walked out to the garage again and put the end of a fat zip-tie up into it. Sure enough, I can slip the tie all the way in and it angles up toward the clutch. Maybe I have a very odd design here. Anybody else with an early 80's R65 can check this out? I guess I could post a pic if needed.