Here's what you need to know:

When Colt first standardized the flattop upper (with assistance from Dick Swan of ARMS), the final version had the height of the rear sight with the detachable carry handle about .040" higher than an A1 or A2 upper.  To compensate for this, it was necessary to raise the front sight by .040" also.

Here's where the details become important:

Colt, thinking primarily of their MILITARY sales, and knowing that military rifles are rarely rebarreled, and if they are, are nearly always rebarreled with an identical barrel, decided to go with a design that used a modified FSB and the standard-height front sight post.  This was fine for the military, since the barrel would effectively be "married" to the upper receiver for life.

What Colt did was simply mill the slot on the top of the FSB .040" higher than usual, leaving a "higher shelf" where the post screws into.  The standard FSB forging is used, so the protective "ears" around the post are "lower" relative to the shelf, since the shelf was raised.  These FSB's are marked with an "F" for "flattop", and are stamped into the FSB on the left side (opposite the markings from the forging die).

None of the civilian manufacturers followed Colt's example (until very recently, anyway), correctly understanding that in the civilian world, it is much more common for barrels to be moved around from upper to upper, and that since the same barrel assembly is likely to be used on all types of uppers (not true for the military), it made a lot more sense to use the standard FSB on all barrels, and simply use a taller post when necessary.  This solution is far less expensive and much more flexible, and the end result is identical to Colt's solution (i.e., the top of the post is raised .040").  The protective ears provide exactly the same level of protection as Colt's setup, too.

For anyone NOT in the military, it makes a lot more sense to simply buy a $5 extended-height post than to spend $$$ replacing the FSB, and thereby limiting that barrel only to flattop uppers.

-Troy

Quoted: Many thanks. Bear with me here please ... So when starting from scratch (RRA upper) and building a flat top with a 16" bbl ... I need to order the taller front post from Brownells when using ANY FSB I purchase or ... any FSB other than an F marked Colt ... is that correct? AND ... will the taller post stick up over the protective ears in most correctly assembled builds? Please forgive my ignorance.

With any non-F-marked FSB, used in conjunction with a flattop upper, you will PROBABLY need the higher post (keep in mind that "tolerance stacking" may allow a perfectly fine zero with the standard post, but it is not unusual for this not to be the case).  With an F-stamped FSB, you will need a standard-height post.

No matter which you use (regular FSB & tall post, or F-stamped FSB & standard post), the post will extend slightly above the ears.

-Troy