Firearms often have the serial number copied on several parts. But the only one that is considered to be the "firearm" is the part that matters. It's called the receiver or the frame. It is the central part that will also contain the manufacturer's name and city. All other parts can be destroyed at will whether they have a serial number or not. But the serial-numbered frame will always be a firearm and possession laws will apply and its serial number can never be altered or obliterated.