I picked up one today and inspected it at an authorized dealer and checked for all the inspection marks, M16 BCG, feed ramps, M4 upper mark, etc. and no, say again, no "F" marked FSB. I even asked the sales person to verify that I was not missing something. I did not take a picture, but is was the real Colt deal with no F-marked FSB. That is when I called Colt to try and get the company line instead of speculating. The customer service person seemed to indicate that there could be both and that both had different measurements, but that the non-F marked FSB was designed/redesigned to be proper for that model (i.e., 20" with flattop). She seemed to think that it would be more common to not get the F-marked FSB now with ramped p production, but that the assembly process could result in either part being used, but insisting that both were still correct. I could not get a direct response to why.