This is an uncommon version of the 1st Galleon mark B because it has Gray's Pottery rather than AE Gray & Co, the company name right up until Portmeirion Pottery was created in 1962. Gray's Pottery appears to have been used on backstamps from about 1928 (and in the first advert in 1930). Hence, it's strange that a backstamp in use in the 1910s and at the start of the 1920s is 'reused' in the late 1920s.
It has been recorded on patterns 8554, 8971, 9113 and 9124, which are all designs from about 1929/30.