Thanks for the replies!
With gentle rubbing with a frayed toothpick while keeping it wet, I was able to get a date of 1898 but the coin was packed with some buildup that all the old coppers I find have on them. I tried a brief (like 3-4 second) ultrasound on it and that removed some of the tightly adhered crud but I saw a green pit where I'm guessing the vibration knocked off a small patch & left the green oxidation XR7 mentioned. I now have it immersed in olive oil. It's not valuable o'course but if I find something colonial or of rarity and want to clean that, I'll want to know best how to do it and not damage a historic piece.
FWIW, I've read that electrolysis on pennies is not a good thing so I haven't done that but I have been having fun removing rust with it and turning old thickly rusted handmade nails & spikes into easily recognizable items. Just ordered some Titanium strips for this and it'll be less messy than using sacrificial iron. Is there any good way to use electrolysis on pennies, copper coins/buttons?
Also, as to a rock tumbler for coins & the like, won't the details get smashed in the tumbling? I think of how rough gems come out shiny & polished and think it would do that to the coins, leaving a shiny slug. Am I wrong on this? How can a tumbler not flatten all the details?
Thanks again for the replies.