I believe only the DIAS (Drop-In Auto Sear) is what needs to be registered to the ATF. You can have a FA/M16 BCG, Low-shelf Lower receiver, and FA trigger; None of those actually make your AR15 fire in full-auto/select fire so they are not illegal. The DIAS is what allows Select Fire. The other FCG parts are also not regulated and registered in any way. You can walk into any gun store and buy a M16 BCG or FCG, or buy a stripped lower receiver with the Low Shelf online.
Of course this may depend on your local and state laws as well so do your own research. I have No issues with it where I live. I have Spikes Tactical lower receivers with the Low shelf and M16 BCG's in some of my rifles.