3¢ Brown LL10 Arc
A long, shallow arc from the top to the bottom margin. It cuts through the 'S' of Postage and the 'E' and 'S' of Three Cents and is quite noticeable behind the King's neck.
Research notes:
There is also a sloped line in the left numeral box, to the right of the '3', but it is not yet known if it is constant.