OK, a chromed gun, with no visible markings except a serial number is almost certainly polished (possibly removing the markings) before being chromed. OR the markings were never there in the first place (parts gun) so no help there.
Small point of order, there IS a legal method (for an FFL gunsmith) to replace/restore a serial number. This is done when/if gunsmithing work would obscure/deface or alter the factory applied number. It can also be done when "honest wear" has nearly rendered the original number unreadable. There is a paperwork process, and when done, the number can be reapplied in a different location. SO, there is a legal path, it is just seldom done, and little known.
It is correct that it is illegal to possess a gun with an altered, removed, or defaced serial number, no matter who did it, or when. There is no legal wiggle room on this point. The gun is contraband, and the only legal course is to surrender it to the authorities. Period.
So, not only is it a crime to alter/deface/remove the serial number, it is ANOTHER crime to possess a gun to which this has been done. And yes, the law assumes that if you have it, you did it. SO, even if you didn't personally do it, having a gun it was done to is also a crime. NEVER BUY a gun with no serial number, and if one comes into your possession through inheritance, or you found it in the wall of your house during a remodel, the only legal path is to contact the authorities, and surrender the gun. You might want to have a lawyer handle that, just for good measure, because, simply put, if you have a gun without a serial number, there IS no legal defense.