I have a 5.56x45 Galil Ace that I've already mailed off a Form 1 to make in to an SBR. I asked my engraver to engrave the plastic lower on the bottom of the trigger guard or in front of the magazine well. He responded that he's not familiar with the Ace, but it looks like the upper is the controlled item. Before I reply to him, I want to make sure that I have a correct understanding of the regulations. On every SBR that I've done so far (AR15's and AK's), I've had him engrave my markings on the serial numbered receiver. However, I do remember discussion of people marking SBR's in other locations such as the barrel. I also remember remarks that the downside of that is that if you remove the upper from the lower, then you couldn't put another SBR upper on the receiver because the required marking wouldn't be present. Makes sense, but I gathered from that that the required markings do not have to be on the serial numbered receiver. Is this correct? Since I would never separate the plastic lower from the serial numbered upper receiver of Galil Ace, would I satisfy the requirements by having my lower receiver engraved? If so, links would be very helpful so that I can email them to him. It seems like this same situation would also apply to an FAL. BTW, if you're wondering why I would want to engrave the lower receiver instead of the upper receiver, the lower receiver location options are much less conspicuous than the upper receiver location options.