Abstract: This invention is concerned with permutation locks having a wheel pack comprising a plurality of rotatable permutation discs, each having a recess which must be correctly aligned before the bolt of the lock can be withdrawn. One known method for determining the positions of the permutation discs is by analysis of X-radiation which has passed through the wheel pack parallel to the axis of rotation of the discs. In locks according to this invention, the opacity of the wheel pack to penetrating electromagnetic radiation along any line parallel to the axis of rotation of the discs is independent of their orientation.