Locks on both sides of the adapter release the M42 mount for full 360 degree rotation.
The locks engage a gear like ring on the mount and lock in small steps.

This of course allows swings and tilts and rise and fall in any position. In addition, this arrangement allows making up variations of thread positions in M42 adapters so the lens can be correctly centered regardless of adapter used.

The two locks are linked together and one is always accessible regardless of adapter position.
The very clever lock ridges are pointed against the direction of rotation and are easily gripped by finger or a fingernail.