Well, you learn something new every day.

I have been looking at assay office Mark's for gold and silver for years. Assay office Mark's are commonly called hall mark and guarantee an item is made of silver or gold. I knew items marked in Sheffield were a crown or a Yorkshire rose, however, I didn't realise that the crown was silver and the rose was gold. I just thought that the mark must have changed on a particular year. As you can see this is different from the other assay offices where the mark for gold or silver is the same.
Incidentally, just in case you are wondering, Britannia silver is finer than Sterling silver and has 958 parts of silver per 1000 rather than the more usual 925 of Sterling silver. I have rarely seen a piece of Britannia silver.