The block has an elongated guide-rail (10) has a groove on one surface extending from rail end to end, and providing access to a profiled chamber (12) running down the core. The profile chamber incorporates a slide-fit holding fixture (21) linked by a shaft-piece (19) to a top-piece (14) and dovetail-fit outside adapter block (15) which slides back and forth along the outer face (18). The top-piece (14) and adapter block can be separated and exchanged by e.g. a dovetail joint as required.
