I do NFA stuff for a living and we process paperwork on a regular basis. So I like to think that I speak with some authority on the subject.
Contrary to what's listed above here's the straight dope:
Let say you have a Bushmaster stripped lower and wanted to registered it as an SBR on an ATF Form 1.
Your name, your trust, LLC or Corp name, will be entered in Box 3B. You will need to engrave this also with your city and state either on the lower or barrel of the firearm. Box 4A will be Bushmaster. For 4G it's customary to enter the existing serial number of the firearm. The instructions state you have to, but there's no legal justification behind that. You can use any serial number you want. If you choose something other than what's on there already you will need to engrave that onto the lower.
In box 4h, I would state "SBR manufactured from Bushmaster stripped lower."
The purpose of having Bushmasters name on the form is to aid in identification and tracing. For example, if someone was the executor for an estate for example, trying to locate and NFA firearm, they know exactly what to look for.
I've seen the several branches of Government claiming to have spoke to the Firearms Technology Branch and that you don't need to add markings. This is in direct opposition to FTB's own statements and letters, not to mention being at odd with what is actually written in the law and is plain wrong.
I suspect in the case of the one on silencer talk, the guy answering the email called FTB and asked, if an individual is making a firearm does he need to engrave markings. The FTB answer would have been "no" as he would have assumed that the question was referring to Title 1 firearms and not NFA firearms. I suspect the .Gov person that contacted them probably doesn't know an SBR from a hole in the ground, hence the confusion.
If anyone wants to argue he point about the marking requirement, I'd be happy to list the appropriate Federal code requiring it.