Did you check the body of the piece for an artists signature ?? I have looked everywhere and have found nothing similar nor has the 618 number ever shown up in the searches !! I would suggest taking this to a good auction house for valuation !! Just don`t leave it there !! I have a concern about the markings on the bottom and there is always the possibility of a fake but I would hesitate to say that it is or is not Rookwood !! But there is nothing that shows up with this form !!
If you find out for sure please come back and update this post !! I am very curious !! Sorry I couldn`t be of more help !!