Dowty crushable sealing washer.
Metal outer with rubber internal seal. As you tighten then bolt the washer deforms and pushed the internal sealing ring against the bolt to seal the assembly.
Check that your fork slider has sufficient space to install the washer.
The outer diameter of the washer is just shy of 14mm.
The ultimate sealing solution.
Sold in pairs.
Last picture shows along side the copper seal alternative.