There are, if I remember correctly, two different flat bottom plate designs. It can be differentiated from other Flat Bottom baseplates by the horizontal ridges running along the edge of the baseplate. The top caps were also designed very slightly differently for these baseplates.
I've come across two variants of this rarer flat bottom baseplate - one has the Gillette stamp and Made in England writing on the top of the baseplate and the other has it on the bottom.
(Shamelessly cribbed from a badger & blade post).