Could of been Seekins that didn't throw it in there, thats who makes them. I recieved my IRMT-R upper last week and it was missing the titanium barrel nut. A quick call to seekins should get you fixed up. Still waiting on my barrel nut but I only called them wed of last week.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: WildBill3/75</div><div class="ubbcode-body">The seekins lower captures diffrently, it's located on the bottom where the pistol grip attaches like the safety spring. </div></div>
4-40 is correct. FYI Spikes and Aero Precision are two brands of lower that come threaded for the "takedown detent spring mod" from the factory. There may be others but those are two I have personal experience with.
If you're gonna do the mod make sure you cut down the detent spring by the length of the set screw (1/8") or the takedown pin will bind.