F marked front sight base = a front sight base where the top part is machined 0.40" higher than the rifle FSB. Colt marks these with an 'F' to distinguish them from standard FSBs.
T Marks. - on T-marked upper receiver every other slot or so had a number in it (T-02 for example). This provides a 'location' so soldiers can return their optics to the proper place (as optics are sometimes stored separately from the rifle due to the use of antiquated rifle racks. T = top. If you look on military issued rail systems the rails have number with the addition of B = bottom, L=left, R = Right (what else?).