For many years, each British assay office changed it's date letter at a different time of the year.
- London in May,
- Birmingham & Sheffield in June, and
- Edinburgh in October.
On January 1, 1975 these assay offices adopted the same lettering cycle.
They used the same date letter and changed it on January first each year.
This means your Birmingham date mark was used from June 1974 through December 31, 1974.

Can't help you with the manufacturer.
Pat.