If I'm thinking what I'm thinking, this is dja1980's old tank!
It's rigged for a hidden closed loop and then sump. The two bulkheads up top that are T'ed together are the CL return, the bulkhead below that is the intake. The top right bulkhead is the drain/overflow for the sump while the bottom right was just an extra hole which was plugged. I think the return for the sump was tubing ran over the top of the tank and not through any of the bulkhead but it's been awhile the last time I saw it.
It seems crazy to do it that way but to me made sense. Drilling into the false wall is better then the rear glass and could be drilled anywhere, on the back glass you want to drill in a way that the holes are uniform to prevent structural integrity if they are drilled too close or slanted from each other.