I won a Stag-10 Marksman in 6.5CM back when Brownells did their virtual shot show. Neat gun.
From Stag's description of the upper it looks like the only thing unique is the fit between the upper and lower (proprietary parts).
" The Stag 10 Stripped Upper Receiver includes a picatinny rail on the top with a Tang Height of .210 Compatibility: The Stag 10 Stripped Upper Receiver will work with our Stag 10 Upper Receiver Parts Kit or any DPMS upper parts kit. Our upper receiver has the same barrel nut threads as DPMS, accepts DPMS barrels, accepts Stag, Armalite and DPMS BCGS and Charging Handles. It also accepts standard AR Forward Assists. Once matched with our custom Stag Lower, Takedown Pin, Pivot Pin and Takedown Pin Detent, this assembly can be built using standard DPMS components. "