F-Marked gas sight base

Could someone please elucidate on this statement for me?
F-Marked gas sight base

Colt's reply included the following directly from Christopher Bartocci's Black Rifle II:


Compared to the standard rifle sight, the carbine rear sight sits higher relative to the bore centerline, in order to maintain enough wall thickness in the top of the upper receiver. These same upper receiver and sight dimensions are also used in the current M16A4 rifle.

Colt initiated a study of the interface relationship between the detachable carry handle and the dovetail on the upper receiver, to determine how this interface relates to the carbine's ability to accommodate the sight adjustment requirements for targeting. Research indicated that the maximum allowable tolerance was well beyond the allowable windage adjustment for the carbine, and accordingly, two modifications were made. First, the locating relationship between the handle and the upper receiver dovetail was improved, which improved the consistency of the rear sight alignment with the bore centerline. The second modification was to increase the windage allowance.

It was soon discovered that some carbines failed to meet the targeting requirements because the front sight post could not be adjusted high enough. To correct this, the front sight assembly was raised up to bring it in line with the elevated rear sight. These higher front sight assemblies were marked 'F' (for Flat Top) on the left side. Both the flat-top rifle and carbine utilize the same front sight post.

Thanks Hellrazor! I hadn't run across that before.