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I posted two pictures of the left locking recess. One is my No5, which is fine, but look closer at the No4, there is a small screw that has been threaded in to fill a hole that has been drilled and tapped. And the screw is protruding right through the locking surface. Click on my imgur link to get a clearer picture.
Go to Post #24 - look at upper rear corner of the recess (upper left corner of the recess in the picture) - the head of the screw must have been on the outside, and the re-finishing job seems to have hidden it well.
If it is the tip of a screw, it doesn't come all the way through. Whatever the blemish is, a moment with a Dremel and it is gone.
If it is a screw that came through from the left receiver sidewall, it is very well hidden externally. Hardly something that would have been done in a military workshop.
It looks like a crap side scope mount drill and tap job. The hole is slightly deeper than the screw as the screw is not bottoming out. Regardless need a pic from a different angle.