This is an exception to the normal rule. It is not a registered design number, but in fact a registered trade mark number, which is a different registry and totally different sequence of numbers. The trade mark of Spode Impl was registered on 11 April 1890, with England soon added.
Impl is for Imperial; an ivory coloured earthenware first introduced about 1821, it is a fine body that can be translucent. Tower pattern was often produced on Imperial body.
Regards
Andrew