from which the probes 74 are formed is only a little less than the diameter of the bores 62 in which they are received hence at the locations where the transverse openings 56 intersect the bores 62 the probes 74 protrude laterally into the openings 56 and thereby compressively engage the flat surfaces of respective brake segments 80 this engagement causes the segments to provide a frictional braking force which prevents longitudinal motion of the probes during the time when the probes are displacing their respective lock tumblers but when the tumblers bind the frictional threshold of the braking force is exceeded and thereafter the probes slide against the flat surface of the brake segments to avoid moving the tumblers beyond their critical lock opening positions to achieve the correct braking force the hoop tension exerted upon the segments by the hoop clamp 82 is adjustable to prevent the hoop clamp from impinging upon the holder 50 in the region between the brake segments 80 the radial thickness of the brake segments is chosen to exceed the radial depth of the transverse openings 56 so that the brake segments project laterally outwardly from the holder 50 as illustrated in fig2 by the projecting portions 84 referring to fig1 and 7 the clamp adjustment mechanism is conventional and includes a worm gear 85 which is captured within a.