In the jewelery industry this is quite common. Use a good hard grade dyed resin or epoxy paint. Clean the mark real well,apply the resin/paint and make sure it gets into all areas by rubbing .Wipe off most of the excess with a smooth quick wipe. When almost set up clean off around the mark with a solvent dampened cloth,finishing ,again,with a quick wipe over the mark.Let dry completely,and finish off with a light polishing/hand buffing to remove the hazy film left on the area. It will show boldly in the color of the resin.This works best with a deeply etched or engraved mark,as the etched surface holds the resin tight. If the mark is stamped,make a deep mark.The resin I use is called Ceramit,and is a two part enameling compound.It comes in a complete kit,or you can get individual color bottles and a bottle of catalyst.Available from most jewelery suppliers.
Stacy