I can't find it on the site anymore, but PSA used to sell this RE as a stand-alone item:
https://palmettostatearmory.com/psa-...-tube-kit.html
I have a couple. Any lower I assemble gets one of these REs first, and goes to the range with a pistol-length upper to shoot a little video of it functioning as a pistol. Then I swap in the RE I'll use for the configuration I actually intend. As noted, I then forever have the option of reconfiguring that lower to be a pistol.