imported post
If your shotgun does not have a manufacturers serial number, I would obtain documentation showing that the manufacturer never placed it on the gun, and keep it with the shotgun.
If you can not get that , another option would be the following. The Calif penal code section 537e requires items have serial numbers from the manufactures intact ,and not covered up or filed down or obliterated. Their is no requirement that it has to be obvious upon casual inspection.You do not have to mar the gun with a hand engraver(or use your DL number which identifies you). You can have a very small metal plate stamped with any number or letter series( maybe including your address so it can be returned if stolen) for 10 bucks or less at most sign shops and rivet it(permanently attach) on the frame in a inconspicuous place, like under the butt plate that is not obvious to casual police inspection(preventing ID of yourself), but meets the letter of the law for having a serial number.