I don't have my digicam with me. The thing the filler cap screws into is a plate about 5" in diameter. Besides the filler hole it has a couple of smaller holes with grommets that the coolant pipes pass through. If you hold the coolant pipes where they enter the reservoir and twist them about a vertical axis, you should see the bigger plate rotate on the reservoir. It won't rotate much because the plate has a slot that fits around a positioning tab on the reservoir.

It could be that they changed the design of the reservoir between 90 and 91, or mine is a redesigned reservoir.

Another area that might leak is the washer fluid level sensor. I'm not sure if it's a distinct unit or part of one of the pumps.

Sixto
91 300SE
81 300SD