That hole at the back does provide venting ...but it supposed to leak
Because it's a poorly designed automatic chain oiler .
An engineering idea who time has passed ... block it off .
The primary needs a small amount of venting
Blocking it off may just move the leak to the next weakest spot
Depending on the year ...
The primary side cover
may already have these .. on the side
vented inspection caps https://www.bsaunitsingles.com/item.wws?cpubcode=BSA&sku=70-9800
The Bsa Unit Singles site says they have 40 in stock