I recently purchased a Queen Ann rat tail silver tablespoon with partial marks. The date letter is clear for 1709/10 and the London town mark shows lion ehead erased. The makers mark as well as the Britannia marks are rubbed. The spoon was sold to me as a Britannia standard silver item. Is there any way I can tell if the silver is up to the Britannia standard? The spoon did not cost me too much more than a Geo. 1 or Geo.11 example. Thanks for your help.