As per my pic rock83 is correct. The brass plug is a block DRAIN. The stock water feed for the turbo is a nipple coming off the turbo specific water hose. It is the second rubber hose coming off the bottom radiator.The engine was rebuilt before I received the truck. It has a new timing cover. I did a compression check of all cylinders and adjusted the valves. Is that what you where told or know proof positive? Shop recites?
On the driver side of the engine block (turbo models), right in front of the bell housing, there are two wholes drilled and tapped in the block. From what I could tell they are the factory oil and coolant supply for the stock turbo.