Why do we have to custom engrave a lower but not a suppressor? Aren't they both considered a firearm by the law? Not that I am arguing we need to engrave our cans. I suppose a can isn't a receiver but logic and letter of the law has never stopped the government before.

If you make your own suppressor on a form 1, you DO have to engrave that info, I've done it myself.

If you buy a commercial suppressor on a form 4, the manufacturer already engraved their info.  Just like when you buy a factory SBR or SBS on a form 4, you don't engrave your personal info on those either because you didn't make the NFA item.

I did not know that. Makes more sense now. SO it has nothing to do with it being an NFA item, but the fact you manufactured or re-manufactured a "firearm".  

Correct, the burden to engrave identifying info and location where it was made is on the party who makes the NFA item, meaning, completes the final configuration of parts into an NFA configuration firearm.

This is also the distinction between a Form 1 (Application to MAKE and Register..) vs. a From 4 (Application to TRANSFER...)

This ^^ If you are not a licensed manufacturer you can't manufacturer a firearm.  As a non-licensee you "make" a firearm.

Look at the title of the Form 1. "Application to Make and Register a Firearm."

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 firearms manufactured, 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

It has everything to do with it being an NFA item. If you make and NFA item, you must engrave your info.