The one I am looking at right now doesn’t even have the name of the original manufacturer on it at all, I did use the original serial number though.
Being you didn't manufacture, make or create the original firearm, you are still required to put that information in 4a.
For example, You have a Colt M4 carbine, and wish to file a Form 1 as an SBR:
-In 4a, "Colt" and their address because Colt is the original manufacturer, YOU are not the original mfg.
-4b, c, d, e, f, g, as described above.
Your "makers" marking is what is in 3b (the name/city/state of your trust that you will engrave on the receiver)
If you are creating a new firearm, not from an existing firearm:
-In 4a, you list the makers name/city/state
-4b, you choose "SBR"
-4c caliber
-4d the model, if you have an 80% that is already marked with the model name.
-4e & 4f the bbl & OAL
-4g Since you are the maker if the firearm, you must add a serial#. You cannot have used this serial# on a previous firearm you have made.