As Alan stated above all the info can be found in the link above. Some important points from that thread. FBI guns are in many variations. They can begin with a variety of prefixes. The TEU ones are of the first batch and will likely letter as FBI guns, but not agent used guns. Later guns that were either used by agents are sent to the academy to be used and then sent back can have a variety of prefixes. Some FBI guns have checkered front straps, some have just the vertical grooves, and most have the checkered trigger guards. The warning on the slide can also be an indicator but it has to be the right warning "Caution Capabale of Firing with the Magazine Removed" or not have a warning at all. That's a start but not all the info. Rarely do LNIB or NIB guns letter as FBI guns but anything is possible.