ButterBolt:  It is all done with friction.  When the lever is pushed to the right, (toward the 'center-line' of the barrel/sight base), it 'rides down' on an inverted screw's thread.  This clamps the two parts of the sight base together.  If parts are clean, it does not take very much effort to move the lever to lock or release.  (Attached picture shows 1901 sight from the bottom with lever in 'locked' position). 

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