Cajun,
You oughta know by now that finding the exception to the rule another member just posted is sporting on this forum.

Additionally, I'm curious if your generalization is in federal law and if so, whether it can be over ridden by state law as the Revised Code of Washington (RCW) appears to do by stating possession of a gun that's serial number has been removed is "
prima facie evidence that the possessor has changed, altered, removed, or obliterated the [serial number]".
RCW 9.41.140: Alteration of identifying marks
No person may change, alter, remove, or obliterate the name of the maker, model, manufacturer's number, or other mark of identification on any firearm. Possession of any firearm upon which any such mark shall have been changed, altered, removed, or obliterated, shall be prima facie evidence that the possessor has changed, altered, removed, or obliterated the same. This section shall not apply to replacement barrels in old firearms, which barrels are produced by current manufacturers and therefor do not have the markings on the barrels of the original manufacturers who are no longer in business. This section also shall not apply if the changes do not make the firearm illegal for the person to possess under state or federal law.