When doing custom machining on a receiver, is it legal to remove the address of the manufacturer from the receiver?
If it is illegal to remove it, is it legal to remove it and engrave it back on at a different spot?
Short answer, no it isn't legal to remove the required markings on a firearm. Your second question is answered by the first. Since the last administration declared gunsmiths fell under ITAR they were determined to be "manufacturers" and thereby bound by CFR. Further reading regarding the illegality of removing or altering of required markings is covered in the text of the Gun Control Act of 1968 (where serial numbers became mandatory instead of an indicator of contractual fulfilment).
The down and dirty is covered here:
27 CFR § 479.102 How must
firearms be identified?
(a) You, as a
manufacturer,
importer, or maker of a
firearm, must legibly identify the
firearm as follows:
(1) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the
frame or receiver thereof an individual serial number. The serial number must be placed in a manner not susceptible of being readily obliterated, altered, or removed, and must not duplicate any serial number placed by you on any other
firearm. For
firearmsmanufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of the serial number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch; and
(2) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed), or placed on the frame, receiver, or barrel thereof certain additional information. This information must be placed in a manner not susceptible of being readily obliterated, altered or removed. For
firearms manufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of this information must be to a minimum depth of .003 inch. The additional information includes:
(i) The model, if such designation has been made;
(ii) The caliber or gauge;
(iii) Your name (or recognized abbreviation) and also, when applicable, the name of the foreign
manufacturer or maker;
(iv) In the case of a domestically made
firearm, the city and State (or recognized abbreviation thereof) where you as the
manufacturer maintain your place of business, or where you, as the maker, made the
firearm; and
(v) In the case of an imported
firearm, the name of the country in which it was manufactured and the city and State (or recognized abbreviation thereof) where you as the
importer maintain your place of business. For additional requirements relating to imported firearms, see Customs regulations at
19 CFR part 134.
(b) The depth of all markings required by this section will be measured from the flat surface of the metal and not the peaks or ridges. The height of serial numbers required by
paragraph (a)(1) of this section will be measured as the distance between the latitudinal ends of the character impression bottoms (bases).