Apologies for the poor quality image, but hopefully people can see the stamp clearly enough. The picture is taken from a collection I'm appraising.
In this example the "n" of "cents" in the surcharge is inverted, appearing as a "u".
The variety isn't listed in Stanley Gibbons. Nor is it mentioned in Robson Lowe. Robson Lowe does make the observation that inverted surcharges are forged.
Has anyone else seen reference to this variety, confirmation or otherwise of its status?