Even if it is, it has to be definitive and to the buyer, it has to be agreed upon as such. Hard sell my friend. Maybe more pics might help your case?
I believe that yo be a worn die crack. A 1866 overdate has never been found. There is a 1866 Shield Nickel that has a repunched date but that shows all 4 digits, 2 of each making it read 11886666z very easy to spot. Welcome to CT.
Use this site https://www.shieldnickels.net/ and look at top 20 varieties. It will show what the 1866/1866 looks like. You may have a repunched date but really too hard to tell with the wear.