This has always sort of bothered me so i took it upon myself to do a little digging on the matter. I don't know, maybe i'm the only one who was taken aback by finding a large chip or chunk missing from the oil filter housing drain valve seal collar (see attachment).
From what i can tell, THIS IS NORMAL. I've seen several images of the filter housing drain valve area, and each look like this.
So if you are changing your oil and you notice this little notch, don't be alarmed!
I don't know why it was built that way, but there you go. maybe someone can chime in who thinks they have the answer to that!