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From what I understand, you CANNOT add or change any markings on an SBR once it is approved. You have to have your weapon in its final form before you SBR it.
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For a Title I firearm, you can remove, modify, remark everything except the serial number. You can turn a Colt into an FN if you like, just don’t touch the original serial number.
Under Tile II (NFA), it’s far stricter. You cannot alter or obliterate almost any markings. Serial number, but also manufacturer, model, importer markings, etc. Adding markings of any kind is totally legal assuming you are not simultaneously altering/obliterating/obfuscating existing markings by doing so.
If ATF would accept a blank receiver with the minimum info engraved (your name, location, serial number, caliber), then you still could legally add a repro/retro Colt logo on the magwell. However you’d be creating a lot of confusion downstream and since the assumption would be Colt is the original manufacturer, though it would not reflect this on your Form 1.
Always better to have the gun marked as intended before you Form 1 it.