have seen this myself.
I would Not worry About it..I bought many with this problem in the past from EBay before I decided to started Selling kits.
It is in fact a casting Error and still is going on today.
The small hole ( although looks scary ) is basically harmless.
If your worried About water or dirt getting into the Hole why not simple pack it with high temp rtv
.This way if you ever have to disassemble ,the rtv would be less an obstacle the WB wield.
Hope this Helps
cheers
Bob