Colt tall square front sight post. This is the correct sight post for fixed carry handle 5.56mm carbines, which do not use an F marked front sight.
It is also the correct post for all Colt 9mm carbines and SMGs, both fixed handle and flattop.
This post can also be used to correct aftermarket sight configurations, where a military dimension detachable carry handle is being used with a barrel which does not have an “F height” front sight base.
