Kevin, not to nitpick but on a headstamp the letters and figures you see are a raised portion on the bunter (the negative). Means it will be hard to “fill in” anything.
Also headstamps are usually made in one of the last head forming operations what means that the data is not applied onto a readily done surface. A headstamp is generated by case material being formed around the raised portions of the bunter until the head reaches it’s final shape. This is the reason why sometimes the raised portions (forming the data) sometimes break off due to the extreme sheer power as the case material is flowing sideways and the bunter surface is (usually) hardened (and sowith brittle). When this happens simply sections of letters or figures are not formed anymore and appear like “weakly printed” or as you say “filled in” - in fact parts of the bunter are missing then.
Of course there are also some (few) headstamps which are stencilled onto a ready and flat case head but this will also not have anything to be filled in.
Long story short, coin stamping and headstamp shaping is two different processes which do not fully compare.