If the owner of a collectible Luger is bothered by pigment loss, I would think it OK to fill those letters that are completely free of older material. A close match to the remaining color would be needed or it would draw even more attention to the issue.

As far as the original material, in Gortz & Sturgess p. 1060 they cite an Infanterie-Konstruktionsbureau (I.K.B.) document dated 1913 specifying that Gesichert be filled with sealing wax. There is another reference on p.774 to a Mauser spec. calling for sealing wax in the safety marking on Persian contract pistols in 1937. I can't find anything else on this, comments by those with better knowledge would be welcome.

Update: On p99 of The Mauser Parabellum by Hallock and van de Kant, they state the Gesichert mark was filled with white enamel. Maybe it changed over time?