Careful here:
I was contacted by the DA, regarding a case where someone was being charged for possession of a firearm that "had the serial number removed". They wanted me to see if I could bring back the serial number. Which is possible in many cases.
I got the gun only to find out that it was a Remington Model 721. Most Model 721s don't have serial numbers. I've only seen one that doesn't.
I explained this to the DA telling him he needs to dismiss the charges, when did happen and the rifle was returned to its owner.
DA's, Prosecutors and most lawyers don't know diddlie squat about guns.
I have several older guns without serial numbers, A model 721 is one, a Savage 24D is another. They didn't come from the factory with serial numbers and are perfectly legal. Even, as in the case of the 721 some did have serial numbers.
Another problem the same serial number may be used on multiple guns.
The Mauser comes to mind. I was contacted by a defense attorney because his client was being charged possession of a stolen firearm. The client swore that the rifle in question was given to him by his father who brought it back from WWII.
I don't know if the story of the bring back was true, but I do know Mauser's may in fact have duplicate serial numbers.
When the Army contract with different companies to build guns, they issue them a block of serial numbers to use.
Often Germany didn't. The contracted with companies to build mauser and they (not always) came up with their own serial numbers.
I presented my findings to the DA and again the case was dismissed.
The above two cases may be rare, but they do occur. If pays to find an expert before caving.
I don't mean a gun nut, I mean an person that has been Accepted as an Expert in your court system. That takes a bit to do, as you're raked over the coals by the defense and DAs, to get the Court to recognize one as an expert.
If you run into problems, FIND ONE because as I said, DAs, prosecutors and most lawyers don't know.