It says that you can engrave your makers info on the “frame, receiver, or barrel. If the AR to be SBR’d has a staked castle nut why couldn’t you engrave on the Receiver Extension. My Trust is the typical long ass name varient and its going to be a PIA to mark on the receiver and marking the barrel is out of the question on a 10.3” MK18 barrel.
Of course the markings cannot be less than .003" deep and not readily obliterated, ect. It says NOTHING in the regs about the marking being in plain view although the receiver extension would be pretty open to view except when the stock was collapsed. Would not the RE be a part of the “frame” of the weapon? Another advantage would be the ability to configure back to Title 1 status if future laws get changed.
Anybody mark their RE successfully on a Form 1?
If the laws change or you decide to remove it from the registry how it’s marked is irrelevant. I wouldn’t mark it on the lower receiver extension, but that’s me.
I would say no as the RE is not a permanent part of the lower, even staked.