Removed from one of my lowers when I changed to a dedicated 9mm lower.
Rock River Arms 9mm mag well conversion block for AR-15 to take Colt 9mm submachinegun magazines.
Description can be found here:
ROCK RIVER ARMS AR-15/M16 9MM DROP-CONVERSION BLOCK | Brownells
Asking $100 shipped.
AR-15 Pistol buffer tube with 9mm buffer/spring. $50 shipped.