A separate mark to indicate importation was first added to British silver by the Customs Amendment Act of 1867. This consisted of a letter "F" (for "Foreign") in an oval. It applied to all assay offices. The use of this mark was not taken up universally as it was thought that customers might think the imported wares somehow inferior to British items. In addition the act was badly written so that there were loopholes allowing normal hallmarking to apply even to imported goods. In particular the assay office had no power to question whether an item was of foreign origin.
The Hallmarking of Foreign Plate Act of 1904 reformed the system and introduced distinct import marks for each assay office. London's use of the sun mark conflicted with various trademarks and was changed in 1906 to the astrological sign for the constellation Leo. Other assay offices' marks also changed in 1906 including Sheffield's which changed from two pairs of crossed arrows to the astrological sign for Libra.
In addition to prescribing the town symbols for imported silver (and also for gold) the 1904 Act also predicated the use of a numerical value for the standard in place of the lion passant for sterling silver and the crown for 18 carat gold. The import mark for sterling silver therefore also includes the value "·925" in an oval punch. The standard date letter for the current year at the assay office is also used although not necessarily in the same punch shape.
A 1998 amendment to the 1973 Hallmarking Act finally brought an end to use of the individual Assay Office import marks so that the same marks can now be struck both on imports and on UK-manufactured articles.
"Foreign" mark used by all assay offices 1867-1904