The early M16A1 rifles set themselves apart from modern M16 and M4 rifles with their front sight posts and barrels. Most modern military 5.56mm barrels have a 0.75" journal, but the Vietnam era barrels were "pencil barrels," with a 0.625" outer diameter and gas journal. When the flat-top M3 / M4 and M16A4 upper receivers came into being, the height of the front sight post changed, and the 20" barrels became "F" height, and the older A1 FSBs were "standard" or "non-F" height.
If you need the sight pin, spring and detent, the round top with the 5-sided tabs is correct for the A1: A1 Vietnam era sight pin kit
Note: this is a part for an experienced armorer or barrel smith to install. Front sight posts are properly mounted to barrels by drilling both. There is no standard to pin placement that you can rely upon, so both the barrel and the FSB should be new and free of pin holes. Also note that the correct holes are tapered, or angled, and not a straight drill through. Installing an FSB is actually quite difficult to make to sure to maintain absolute 12:00 dead center aim of the sights. This sounds easier than it is. It is one reason that we send our FSB work out to the shops that have the right equipment. If send it out, so should you.
