It looks like two coins joined together. Have 2 coins been hollowed out to make a box? If so you might find it unscrews or pulls apart. Does it ring when you tap it or is it obviously a dead sound? It isn't right, but doesn't look like an error because the toning is different on either side of the line - left side is lighter than the right, so it started as 2 coins.
I guess I should have explained the picture a little more...the first picture is two coins side by side to compare widths. The coin on the left is a 1936 Penny and the one on the right is the one in question. No idea what year but I believe it is a Queen Victoria. I do not know anything about these coins, so it may just be that the older pennies were smaller and this is just tremendously worn possibly a pocket a piece.
I have a couple of pennies that are thin like that. If you look very closely the diameter will also be very slightly under size. These penneis have come about by some one dripping them in acid. Taking the thickness down by a half a millimeter will reduce the thickness considerably whereas half a millimeter of the diameter is hardly noticable.
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I do appreciate the insights/thoughts.