I dont think it is water jacket, I think it may do with oil? there is 3 identical sized hole along inlet side of blobk 2 go straight threw to sump wish it had fell thre one of these but it fell threw the 3rd which seems to have a bit of black plastic on gearbox side dunno if this removable? Will get a better look when block out got halfway today and got to cold to lying on drive pml