You may have seen marks called Registration Diamonds on various things before. They were a devise used to show that the design of the item (be it pottery, glass, metalware etc) was registered by the manufacturer. The format of the Registration Diamond is very specific and can tell you a great deal about a piece if you don't have any other clues to go by (such as maker's mark) and if you know where to look for the referencing information. http://www.thepotteries.org/mark/reg.htm is a great resource to give you info you need to decifer pottery registration marks. Using the info this website provides, i can tell you that this Conway patterned plate by P.B&S was registered on the 6th December 1880. Why not look at your own collections and try it for yourselves?