[lugerholsterrepair]

Rich, I would guess a squeegee approach would be best. I had an extensive conversation with a Luger restorer who does this and that's how he did it. I wouldn't get too agressive with residual paint left behind but let it stiffen up and peel or scrape it (gently with wood or plastic)when it's dry. Just be sure to squeegee off the letters leaving them full of enamel. A rubber spatula perhaps?
If I were doing it I would remove the safety lever for a clear flat area to work. The fellow I know did a spectacular job, matched the color exactly too. He did this after bluing..

One other approach I might consider if I were doing it is a hypodermic needle. Use a very fine needle and gently ease the paint into each letter. Might work better than the squeegee method..