I've heard back from the Devon & Cornwall branch of the Institute of Quarrying and they do not have any records of the Devonshire Oilstone. Also, the Mindat.org entries for the Wheal Jewell and Friendship mines does not list an oilstone as one of the products removed from the mines. However, seeing as the one thing we know for certain about this particular stone is that it wasn't ever widely distributed or an important item for local commerce, this isn't as big a setback as it would seem.

I've been given some other contacts to ask about the stone, so I'll see if any of these pan out.

Regards,
Alex