Remember, a Form 1 is for "making" an NFA firearm, not "manufacturing". On the Form 1:4a.Name and Address of Original Manufacturer and/or Importer of Firearm (if any)
"(if any)" would apply to the guys making a silencer from an oil can, Maglite or making their SBR from an 80% lower......where there was no original firearms manufacturer.
Your Responsible Persons Questionnaire asks: 4b. Name of Maker, Manufacturer and/or Importer of Firearm.
You should include all that is applicable.
If your making an SBR from a Colt M4 Carbine, "Colt" is the original manufacturer. You (or your trust) would be the "maker".
If you are making a silencer from a Maglite, then you leave 4a on the Form 1 blank or put "none". On the RPQ you just don't list a manufacturer.
If you are SBR'ing a Yugo AK it would be Zastava as the manufacturer, Century Arms as the Importer and you or your trust as the maker.