I ran across this photo which shows more clearly that the two edges which contact the underside of the barrel are rough. It appears to me that there are teeth intentionally made into the edges. These teeth are slanted such that they would grip the underside of the barrel when the lug is pushed back towards the bolt, and would release very easily when it is pushed toward the muzzle end of the barrel. Also, take another look at the two lines on the lift done by Day. They look to me like impressions left on the underside of the barrel by similar teeth could have formed them. (Where's a forensic dentist when we need one?

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At any rate, this seals the deal for me. There is no mystery of the two lines anymore for me.