You'd be surprised at the people who like to argue this. When you change the configuration you are not the manufacturer, no engraving needed.This is only true if the firearm isn't already engraved with manufacturer markings. For example, you made your own from an 80% lower. If it already has markings, you can "adopt" those markings and no additional engraving is necessary. This is spelled out clearly in the guides put out by the ATF.