Laws in the United states should be read exactly as written, because they mean exactly what they say.
And yet, "should" is not "must". According to the courts.
And often, laws are
not clear, do
not mean what they say, and are
not enforced or prosecuted according to what they say, but instead based on what regulation has been created based on that law, even if said regulation does not match the law. Or based on what the particular law enforcement individual needs the law to say to gain leverage or traction in a given situation, whether it will survive a court case or not.
Whether or not I agree with any particular person in this thread---the safest way to ensure a lack of legal issues is to place some sort of serial marking on said firearm. Whether that is
required is a separate issue.
I realize that I'm not actually answering the original question from the OP (or even later variations of that question) but there seems to be an argument going on in which people are arguing about two different things---one of which is what the strict letter of the law says (which, unfortunately, is KNOWN to be as grey area), and what MAY happen (which, unfortunately, is based upon whether or not some official person wants to give you grief about something).
The difference between "should" and "must" plus the lack of official definition of "manufacturer" (as opposed to "homebuilder") means the the grey area is something that can be used against you.
IMO.