The center shows two figures, one wearing a crown (king or queen?) and they're holding hands -- dancing, perhaps?? Is the dance a metaphor for the English Protestants inviting Prince William of Orange to become their king? The decoration includes roses (English?) and tulips (Dutch?). Surely not William and Mary themselves? It has the form and size of an alms dish, but the absence of wear suggests it was a decorative piece for display or political statement.
Details and more photos.