Abstract: An axial tumbler type lock having a split cylinder within a stationary lock casing contains a set of regular spring-urged split tumblers which bridge the shear line to render the lock active and which are depressed to the shear line by a tubular key to allow turning of the lock to a release position. A modified split tumbler with a reduced forward pin extension also bridges the shear line of the lock and responds only to a special key having a forward pin extension for depressing the special split tumbler to the shear line. Additionally, the lock possesses one or more hidden tumblers in the non-rotational lock cylinder section which are spring-urged and have the capability of moving forwardly into the pocket for the special tumbler or into the pockets of any of the regular tumblers if the proper key is not employed to depress the special tumbler to the shear line, or if the rotational section of the lock cylinder is somehow turned without the proper key in place.