Questions about "F" marked FSB on AR-15...

[mgregg85]

I just bought my complete upper from M&A parts, I kinda figured it would be cheap and not the best quality but i've been surprised so far. The upper has the proper M4 feed ramps and the carrier key appears to be properly staked.

My question is about the FSB, this is a flat top upper and the FSB isn't "F" marked but on the right side it says "<B>1". I've seen that marking on some FSBs that are "F" marked and I don't know what it means.

Any ideas?

 

[Onmilo]

The F designates a tall front sight tower used to regulate A2 sights with carbine length barrels.
The <B> would be a forging die code or manufacturers code.

 

[sv51macross]

I thought that the F stood for Flat-top, being that most all rear flip-sight makers design for the F-marked front post.