What does the letter x mean at the end of a hallmark?

by Louise
(Blackpool UK)

I have a pair of 15 carat gold cufflinks that are quite some years old. The hallmark says 15 .625 Anchor symbol X. I know this means 15 carat and I believe the assay office is Birmingham. What I don't know, is the year. Which year does X signify? Please help!