Where is this manhole cover?

I'd seen on Facebook that Wiesbaden has these really interesting entrances to the sewer system. I don't know how old this picture is, or where this is supposed to be, but apparently Wiesbaden's sewer system (the Salzbachkanal) was a tourist attraction in Imperial Germany. [Source]

I tried looking for it, but the best I could do was this big metal square. It seems about the right size, but it's not the right shape. Maybe I'll come across it later, but for now it remains a mystery.