Unfortunately, the ornate design of the dish makes it almost impossible to read the faint hallmark, and it doesn't scan easily, but in the hope that someone might recognize it anyway, I include a 1200 dpi image of the mark, and a link to a lower resolution image of the dish itself.

I see, from left to right, a faint crescent; the crown; the fineness "800" stamped upside down; and then, I see what appears to be an imperfect stamp of a circular monogram of some sort, perhaps containing a Fraktur capital letter K or R, perhaps inside a Fraktur C or E...but I'm just guessing.
Here is a link to an image of the dish itself, which contains a central cartouche depicting naked babies or cherubim; I believe one is waving a veil or standard of some sort, and another appears to be blowing on a conch shell; the figures may be depicted floating on a sea-shell on waves, but I'm not sure.
http://www.concentric.net/~rsk/Ag/dish2.jpg
The hallmark is stamped under the (viewer's) left-hand edge of the dish as viewed in the above picture.