Abstract: A seal has a fiber optic bundle which passes through a fastening device. One end of the bundle is exposed to a light source and the light pattern emitted from the other end is observed to determine whether anyone has tampered with the bundle or the fastening device. The bundle has end fittings which are received in intersecting passageways formed in a block and which pass through each other at the intersection. A flat portion of one end fitting passes through a one-way push nut integrally formed in the other end fitting to interlock the fittings. Alternatively, one fitting is press fit in the block and the other fitting is engaged by a one-way push nut mounted in an enlarged portion of its respective passageway. The shapes of the passageways correspond to the shapes of their respective fittings to prevent rotation of the fittings and to align the fittings for intermeshing with each other. The fibers in the bundle are divided and extend through divided passageways formed in one of the end fittings.