Don't forget the shoe polish, the old internet lore was white shoe polish filling in sketchy stampings to hide what the bottom truly looks like.
I think what is used also depends on what you're trying to fill in. The deeper the stamp, the better your results will likely be with lacquer or enamel. For example, a stamped S/N on a Russian filled in with heavy white:
Chinese S/N and arsenal stamps:
Owen, the Yugo you have in your example I think is possibly unfilled. A light pentograph/CNC electropencil mark like is typically seen on the Yugo receiver or a light mechanical or laser etch as is found on the magazine are both quite a bit lighter than a typical stamp:
Actual stamps on these Yugo parts are actually quite dark, the bluing appears to have been performed after the stamping was made: