Yes there's a catch, if it bothers you. They are rollmarked "Piston Carbine" as they're from a LE6940P.
CARBINE CARBINE CARBINE...NOT PISTOL!!!!
https://www.armsunlimited.com/Colt-M4-Carbine-Lower-Receiver-Assembly-p/lr556.htm
CARBINE CARBINE CARBINE...NOT PISTOL!!!!
https://www.armsunlimited.com/Colt-M4-Carbine-Lower-Receiver-Assembly-p/lr556.htm