F marked has a tiny F stamped on the forged front sight base somewhere. It indicates that the shelf the front sight post sits on is like .040 higher than an A2 front sight base shelf. They do this to accommodate a removed carry handle because it sits slightly higher than a fixed carry hand does forged on the reciever.
Most of the time you can still achieve zero, if you can’t you can buy an aftermarket shorter or taller front site post for your needs.
If you’re building a fixed carry handle clone or A2 or earlier design you’d want a Non-f marked A2 FSB preferably.
If you’re building an A3 or later flat top AR you’ll want an F-Marked FSB to accommodate a removable carry handle if one would ever want or back up irons etc.
if you want a free float rail get any pinned fixed FSB model, A2 or f marked etc and cut off the triangle and leave it as a low profile gas block. It’s the most rock solid of ALL gas blocks .
If you want to press out the pins of an A2 and replace with an F marked or just swap a FSB for FSB you’ll maybe have to re-drill and use Taper pins.
