The half moon cutouts may be to make it easier for seal removal.
I was a senior electronics "train driver" when I was told I had to cross-train over to become a floor mechanic or hit the door with no retirement. One thing the instructor drilled into my head is to always verify the depth, ID and OD before installing a new seal. With calipers or gauge, check the area with seal removed. Check the old seal. Check the new seal you take out of the box. Check old seal numbers against new seal. The new box could have the wrong seal. If you see any problems, it makes no sense to install the new seal.
