Abstract: A system, suitable for use as an identity seal for an object, such as a container for fissile material, comprises a body including a cap for fitting over one end of the object, and a recess which contains a material such as "ARALDITE" or aluminium acting as a matrix for a plurality of inclusions of e.g. bronze embedded therein in a random configuration. The recess is closed by a cover which includes a central electrically conductive part. Also embedded in the matrix material and constituting further "inclusions" are two piezoelectric crystal transducers each electrically connected to the central cover part. In use, the transducers are electrically connected via the central cover part to external ultrasonic testing apparatus to provide a unique output indicative of the particular structure of the seal particularly the inclusion configuration and hence providing a unique "identity" and hence indication of integrity for the seal.