I also have similar coin in the inheritance from my grandfather, more info and photos I posted on PapersOwl. My coin is black and I know that it appeared about at least 100 years. It will be luck a lot if anybody to tell us more.
It's got a lot of patina, my guess copper, but I'm no coin expert and don't know a lot about the large cent / penny, never even saw one, so I can't help ya sir, wish I could... "D"
I'd keep cleaning - any copper I find that is that toasted is worth noting more than copper scrap - doesn't even matter if only 5 were minted - I've been using a dremel tool with the buffing wheel attachment and Mother's Mag Wheel cleaner with some luck, takes 5 or 6 applications but been pulling dates and details off of most LC's
I've used the Andre's pencils. After years or cleaning them, they work best, but once pitted like this one (and identical to one I got the other day), it's almost hopeless.
Dug these two yesterday double large roundness in one hole. Was able to get 1817 on one but zip for date on the other. Only about 60% seem to have readable dates.