http://www.transcollectorsclub.org/index.php

Could be part of a backstamp but more likely,,since your piece is probably English Staffordshire transferware,,part of a previous production run that happened to stick to yours as it was being made !! The link above is for the transferware collectors club that has a good data base for searching by the pattern name !! You could buy a limited 24 hr membership if you want to research further !!