You're right, Anne-Claire, this is a Rathbone mark. It is shown on page 37 of Ian Harvey's book on Rathbone with the caption "This mark is applied in blue to earthenware Chinoiserie jugs in pattern U81. The jugs are the earthenware versions of the porcelain examples marked with the R in a sunburst. The earthenware with this mark is seldom seen and then only on this pattern." Great find, and another great addition for the database. Thanks, Anne-Claire!