I was able to take these photos recently at the Rock Island Arsenal museum.
This is listed as an XM16E1 lower that was used as a testbed for features that would eventually lead to the M16A2. Why and how this XM16E1 lower is fenced is something of a mystery to me, but that’s what both the display and front desk said.
The two most notable features are the “Brunton bump” which is a massive brass deflector built into the upper.
The other is the proto-A2 rear sight. In this version there is a hole in the carry handle allowing the shooter to see the drum setting.
The windage adjustment knob is squared rather than cut at an angle on the end like the final A2 sight.
Man I wish cameras back then had more pixels (or maybe the shot is just out of focus) I can’t see that side attachment point at all, curious where they drilled the rivet hole for the sling swivel lol. Also never seen one of those grenadier sights before, that’s pretty cool too.
Colt makes a little collar that fits on the barrel under the FSB. Attach it to one side or the other, and remove the existing swivel, no drilling required.
Neat, never seen one of those before either, we’ve come such a long way since then lol.