Hi,
Your image embedded; International Convention on Hallmarks (UK joined 1972).

From left to right:
1. Sponsor`s or maker`s mark (OPT).
2. Common control mark
3. Fineness for sterling silver (.925)
4. Assay office looks like anchor for Birmingham assay office
For Birmingham see here:
https://theassayoffice.com/internationa ... tion-marksRegards