there were three of them on pk i was just wondering which one i needed to order for a f marked front sight base 16" barrel
Oddly enough PK actually has four FSPs listed, none of them the standard one, which is what you want if you're F marked FSB will be used with a flattop.
7A Front Sight Post - A1 Round
7B Front Sight Post - Extended +.040"
7C Front Sight Post - Super Extended +.100"
7NM National Match Front Sight Post .050
You can probably use the A1 if you don't mind a rounded post with five adjustment notches, or the .040" extended FSP which you can probably adjust to zero.
Some say the higher sight deck of the "F" marked front sight base is necessary for the shorter sight radius of a carbine length gas system as opposed to that of a rifle while others maintain that the "F" marked sight bases are specifically for flattop/removable carry handle models regardless of the sight radius. Since both explanations make a certain sense, I wrote Colt (via snail mail) to ask the significance of the "F" marked FSBs.
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) [bolding added - MMcfpd] on the left side. Both the flat-top rifle and carbine utilize the same front sight post.
A few previous postings of the above have turned up posters telling me that Colt doesn't know what they're talking about. But there you have it, the best verified explanation I've been able to find.