HK416A5 (and MR223A3) does have numerous changes and upgrades over older variants. Some are easily noticeable, some require closer inspection and some are well hidden. One of this well hidden differences is design of how barrel extension interfaces with upper receiver and how it is positioned.
In my opinion new method whit cut out in barrel extension and wedge on upper receiver is much better idea that classic barrel extension pin in AR15. New design is simpler, stronger, more fool proof and provide better positioning in barrel in relation to upper receiver.