I remember doing this one time, and as I recall I just used a little muriatic acid. You should be able to get it at your local hardware store. I think it is commonly used for cleaning brick. Use rubber gloves and do it outside if possible, cause the stuff and its fumes are really nasty. Just wipe some on the number pad with a disposable rag, wait a little while and if the numbers were simply filed off, they should start to show as a darker color, (though they will be fuzzy and hard to read). If numbers show up, you might as well proceed to the dumpster with the case, at least in my opinion. In the state where I live, possession of cases with serial numbers that have been tampered with is a felony. If the case doesn't show any numbers you need to nuetralize the acid that you put on by flushing with water. The muriatic acid also works for removing rust from hardware, as well as stripping zinc and cad plating. Be especially careful if you use it to strip cad though, as it is my understanding that the gas released from the process is cianide (what they use in a gas chamber) If your case doesn't show any signs of previous serial number, then what? If you stamp numbers in it, you have broken the law by "tampering". As far as I can see, it is a lose/lose situation. Good luck though.