Nice Bruce! I am in Eastern MA and also and found my 1st Mass cent last spring. Fortunately, they are thicker and made of higher quality copper than English coppers and some large cents. Looks like you've cleaned it as much as you're going to get it. Once that surface feature layer starts to peel, there usually isn't anything left.
While we are at it, let me say that I have used mineral oil soaks for years and really don't like it. By accident I found a method that seems to give great results with old coppers:
I carry a small vial (actually a glass spice jar from a spice rack). I put in about 1/2 tsp baking soda and mix with distilled water filling the vial about half way. In the vial I place an oxidized lead hem weight. When I find a copper, it goes right away into the vial--no rubbing whatsoever.
When I get home, I gently clean it with running water and q-tips, wiping gently, until I think the dirt is off. Let it dry for a few minutes, then put on a light coat of Renaissance wax. Let dry for a few minutes (better in the sunlight), then buff with a soft cloth. Sometimes I repeat the wax.
Unfortunately, once a copper is soaked in oil, the Ren-wax doesn't seem to work. I have not tried dipping in acetone or other solvent to remove the oil.
I have been using this method on coppers --including large cents and indians--for about 2months with startling results--I can't save them all, but some turn out gorgeous. Unfortunately, I don't have a camera to post an image at the moment.
I'd be interested to know if others find this method to their liking (no guarantees!).
I suspect that the reason this works is that the baking soda makes the water mildly alkaline, and arrests the oxidation process that occurs when coppers are re-exposed to air. I am not sure but the lead oxide may be reducing the copper oxide (I need a chemist to figure this out). I had found a lead hem weight, dropped it in a vial, and an hour later found a cruddy-looking 1828 LC. When I got home, the LC cleaned up nearly perfectly. I have been a believer ever since.
David