I would not call the dish a fake,, the maker did not try to pass this off as Wedgwood !! But whoever sold it to you certainly did !! Jasperware was and is made by other mfrs !! Odd thing is that this particular mark has not shown up anywhere on the internet !!
One more thing ,, can you tell if this mark is stamped on top of the glaze or is it under the glaze ?? If on top the mark will be more dull than the surrounding area, if under the glaze all will have the same shine !!