Short version, upper receiver barrel socket has a slot, and the barrel extension has a pin that will mate up with the socket when the barrel is installed and barrel nut tightened.

In your case, barrel rotated in the socket against the slot, and the barrel is rotation cocked in the upper receiver to cause the FSB to be canted off to the side.   If you pull the B/C and look at the barrel ramps, will see that the center lug is not TDC to the bottom of the upper receiver.  To add, when a FSB is installed to a barrel, its jigged to hold it correctly TDC on the barrel and the channels are drilled for the taper pins through both the FSB and the barrel.  If you try to install a different FSB using the same barrel channels, best chances if the FSB was already drilled for another barrel, it not going to index correctly on the new barrel to be TDC.



So if this is the FSB that was factory installed on the barrel with cross pins, and the FSB and it channels where drilled/taper pin correctly, then just a matter of slipping the barrel in the upper barrel socket to correctly index it (and the barrel feed lugs).  In most cases, don't need to loosen the barrel nut to slip the barrel, while in other cases where you don't have a stout FSB to use it drive the barrel rotation in the upper socket, may have to loosen the barrel nut to shim the barrel extension pin against the barrel socket channel to hold the barrel in the correct rotation orientation as the barrel nut is being re-tightened.