I have a bushmaster 14.5 inch carbine  with A1 fixed carry handle,The FSB has a crown embossed  on it.   So now I want to build a new upper for it.I have ordered a new 16 inch heavy barrel from Windham weaponry which  also has crown embossed on the FSB . Also ordered a BCM flattop receiver which I'm going to install the new barrel on. And now here comes real question I have just bought a stand alone Troy battle site  ( which is made to use with a standard FSB )to put on my BMC flattop receiver .  I believe I'm going to have to install a taller front site post  in the  FSB.  CAN anyone advise me what the differences between  the standard front FSB  and  the F marked  and the Crown embossed  one ?

"F" marked FSB is .040" taller. Any other markings are forge marks.

Mine only has the crown  embossed the left side of the FSB.

Welcome to the board OP  The crown is just a forge mark which indicates who makes the forging for the FSB.  The old Bushmaster used to have the same forge mark on their FSB and I guess WW uses the same folks.  If your FSB is NOT stamped with an F on the left side, then it's just a standard FSB which was designed to be used with carry handle upper receivers.  You can use the barrel with standard FSB on a flat-top upper receiver, but you'll need to get a taller FS post in order to sight it in properly.

Ha ha I knew there was something to it, thank you