This missing part is not that common but it does happen and the cost for a hand engraved replacement far outweighs its size. I have replaced a number of missing "SAFE" plaques using a simple and inexpensive method that does pass for that is OK looking. Obtain a piece of Brass sheet thick enough to fill the depth of the space it is to go but larger all around, obtain a set of Alpha Numeric metal stamps I found that the 1/8 inch fitted the bill in most cases though there other sizes available, all of reasonable cost. Using the letter "S" punch as close to the center of your piece of brass file the dovetails so the "S" is in the correct center position, when you have obtained the correct fit in the action polish the Brass ink in the "S" with some India Ink. It is not the perfect replacement for the engraved safety plaque and all I can say I never did get a gun back to have the expensive full engraved safety fitted.