Hi Bushpig,
Nice find. I don't believe that it was minted in 1831 no shillings were minted between 1829 - 1834 check the size 23.5mm and weight 5.65grms. It can't be a proof as there wouldn't have been a Queen Victoria die created for the coin as she didn't come to the throne until 1838. Also her image is the second version which came after 1839 were the WW below the neck was removed. More then likely an unrecorded die date error between 1839 and 1863 from then on they put the die number on the coin, Also in 1867 the head changed to young head portrait 3. As to the value to a collector only one that collects error coins would be interested. Personally it tells more of a story if kept by the finder and keeps the line of the provenance to the object.
Hope this helps.
cheers db