The DT1023 will have the F-marked FSB. I will make a note to have that changed. We are in the process of changing our internal software for how everything
is done. The person that normally is working on the website has been busy writing/programming the new system to meet our needs. Pretty much everything that has a flat top will have the F-marked FSB, unless it requires a post-ban FSB. Post-bans have the standard FSB with no bayonet lug. You don't have to have the F-marked with the flat top, either FSB will work just fine. BUT if you use an A1 you will most definitely need the standard FSB since there is no elevation with the A1. The A2 uppers require the standard FSB.
Hope that helps!