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I've never seen a reservoir that looks like that. Early SC400s had it over by the PS reservoir, square shaped, and later ones moved it into the bumper space like SC300s. That one looks like a modification. I can't quite make out the markings, but it looks OEM, from a different application.
i have a sneaking suspicion that the previous owner put a cover there to cover the reservoir and plumbing for the hydraulic radiator fan because that's what is mounted in that location on my car.
Guessing you swapped in the electric fans from an IS300 or similar with the built in reservoir on top. You can use that reservoir instead of the bumper mounted one if you like.
You would just have to reroute the overflow hose from the radiator to that one, fill it, and grab a cap from an IS300 and you should be good to go.
