When doing Form 1, you claim to be the manufacturer of the device and need to engrave just as any other manufacturer. When doing Form 4 you are not the manufacturer but are having an existing NFA item transferred to you.
Many folks that want a SBR have two paths. Become the manufacturer and convert a virgin AR15 lower to an NFA registered SBR OR purchase the SBR and complete Form 4. No engraving is needed on Form 4 because someone else made and registered the SBR.
I understand now, thanks for clearing it up. I have been considering building my SBR, so that might help me decide which way to go.