Looks like Stag Arms requires a FFL transfer for completed uppers.
https://www.stagarms.com/stag-15-tactical-rh-chphs-16-in-5-56-upper-bla-sl-md-stag15100125.html
https://www.stagarms.com/stag-15-tactical-rh-chphs-16-in-5-56-upper-bla-sl-md-stag15100125.html