"Maker" and "Manufacturer" are interchangeable according to the "Definition of Terms." The "Manufacturer/Maker" of the Title I host gun obviously already has their info stamped on it. If you "Make" or "Manufacturer" an NFA firearm, which you are doing by definition of a Form 1, you need to include your info on the receiver as well. It does not matter how the gun started life, complete gun or receiver or 80% non-gun, with the Form 1 all things are made equal as a new NFA, Title 2 firearm is being manufactured or made and the Maker's info in required. The law and regs are really very clear here.
A letter from NFA Branch is valid ONLY for the person it is addressed to, it does not give permission or approval to anyone else. The gentleman who received an NFA Branch approval letter telling him he did not have to include his info on the HK94 SBR or SP89 is protected in not including his info on the newly created SBRs, all others are not. Letters, phone calls, e-mails messages can be mistaken and taken oout of context. Approval letters from NFA Branch have no legal requirement to be legally correct, though the holder of such a letter has a bonafide defense against prosecution for acting according to the letter's approval. This legal protection does not extend to others. The Law however cannot be taken out-of-context and the law trumps an approval letter every single time. If you act contrary to the law and you rely on someone else's approval letter, that letter will not be admissible in court in your defense. That's how it works.
Just engrave your info. While it might seem like a small thing, if you have not followed the regs, your guns can be taken from you permanently as contraband. Prosecution of not, it's just not worth it.