I think, maybe, we don't need to talk about any relationship of close timing between up and down piston movement and hole alignment in the breather. The hole is only aligned every other crank rotation. I can't believe it's been almost ten years since I mocked this up to see what was going on, so my memory may be weak, but I believe the breather is open at 90 deg ATDC. I have guessed, right or wrong, that the breather is trying to keep crankcase pressure as close to one atmosphere as possible so that oil can be pumped in and sucked out. This may disagree with those who believe (or know) that there is some performance benefit based on where in the stroke the breathing occurs. By my theory, such benefits (or detractions) would be purely coincidental with respect to BSA intentions. (I can be wrong and often have been.)
Then there is the matter of why a lot of oil spills out when there is substantial wet sumping. Seems to me it is way too much to get in from the back side past the flange. That means it would need to enter past the tapered end in the portion contained by the outer cover. There is an oil drain hole that could allow this. So, it seems that the wet sumping would have to fill the crankcase about up to the height of the breather for oil to get in through the breather's open end. (Waiting chastisement for silly or "Richard, are you just now realizing this?" Well, not quite just now.)
Richard L.