[Ted Brown]

This is kind of an interesting conundrum. SEI, Armscorp and a couple of others used a rear lug that is basically very close to the original mil spec 9352637 dated 21 Jan 86. LRB Arms lug is similar, but extends slightly lower than mil spec. SAI uses a lug that is completely out of spec and does not include a screw hole for a torque screw. It's kind of interesting that the Gov't. drawing mentioned doesn't include a hole for the torque screw either.
At any rate, they all seem to do what they are intended to do when bedded properly.

 

[John Coleman]

My first lugged receiver was a cast SEI receiver. The lug was threaded for a screw. I then bought an SAI lugged receiver when SAI first started offering them. The lug had a much larger (oversize) non-threaded hole in it. A few years later I was told the larger, non-threaded hole was designed for a flanged bushing that fit from the top of the lug hole and was was threaded in the center for a screw.

 

[John Coleman]

My SAI receiver lug had fairly rounded corners, very rounded really compared to my SEI receiver lug. My SEI receiver lug was not very rounded at the corners at all. Some information from Gus Fisher here.

http://m14forum.com/gus-fisher/62658-shape-rear-lug.html