Pretty sure what you are seeing is a handguard with the original serial number XXXX’d out. They do this when they take a serialized parts from one rifle and put it on another rifle. Sometimes they will XXXX out the original number and stamp the serial of the new rifle onto the part. This is called “force matching” or a “force matched” part.
Can’t find a pic of forced matched AKM part but here’s some forced match Tula parts. Same idea.