Kind of a gray area for the lower? While not considered the "firearm", it is drilled for a sear. It is an earlier lower with the round opening for the mag release and ears for the front trigger guard pin. Can't tell if the carrier has the sear trip.
https://www.gunbroker.com/item/814188151
Not a grey area at all from what I see. As you said, the lower is not the firearm, the upper is. The lower is just a firearms part, like any other. No different than having a FA BGC for a AR-15 or a FA hammer or selector for a FNC. Now if you have this lower and all the other parts needed to make a FA FNC (assuming the sear was not registered), you could possibly fall into the "constructive possession" of an unregistered NFA Title II firearm, but just owning the parts in the auction wouldn't get you there. FNC auto sears are not "drop in", so just having a receiver cut/hole for a sear isn't enough. Still need an auto sear, sear spring, sear pin, FA selector, FA hammer, FA bolt carrier, FA safety sear (Sear B).
The "issue" with an auto sear hole being drilled in a AR-15 is that you are modifying the
actual firearm (since the lower receiver is the firearm on an AR-15) for full auto use. In the case of a FNC, having an auto sear hole does not cause any FA specific modification to the actual firearm (i.e. the upper).
