Kev, I think the point I'd stress is that no where does it say that you can or cannot engrave with or without a FFL. The point made to me was "are you taking possession of the firearm?". If the answer is yes, then you need the FFL. If the answer is No, then you don't. Regardless if you were fixing the sights, engraving Trust information, or making any other modification. The point of the FFL is to keep track of the movement of weapons so if anything is stolen or a crime happens, then they can trace that to it's last known point.

Gary, yes, a private party doesn't log it out, but they are also the ones that hold the Stamp that allows them to have possession of it. You cannot possess a weapon that has the stamp, unless you are, 1) Listed on the Trust and have a copy of that trust on your person, or, 2) Have a FFL, and have logged it into the books.

If it's stolen from your shop, they are going to the person that brought it, he/she is going to say "I dropped it off at Gary's", and then the ATF is going to show up and want to see when it was brought to you. You'll have zero proof of it ever being in your possession and you'll have an issue at that point.