So the 12mm M6 I needed had a plain top (see screw on the right) so it couldn't be tightened like the left one which I had to buy separately. My guess is that this is what happens when you process millions of screws. It's also not a good thing that the up and down tilt adjustment doesn't use locking nuts, so you'll have to use a screwdriver if you want to adjust the viewing angle (up and down). .
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