Numerical purity, such as 800, 830, or 900.
These numbers identify non-UK silver for instance, that 800 is 80% pure silver. Very common in older European items in German, Italian, and Greek silver.
People usually see 835 silver on vintage Scandinavian, Danish, Norwegian and Dutch silver.
Each country stamps its own national mark; for instance, Germany shows a crescent moon and a crown next to a fineness/purity number like 800.