Abstract: A block having first and second permanent cavities. The first cavity includes a first vertical, continuous V-groove from the rop of the block to a pocket which has a permanent horizontal bottom wall. The second cavity includes a second V-groove that is parallel to the first V-groove, and which is continuous from the top of the block to a second permanent horizontal bottom wall that is at a predetermined height with respect to the bottom wall of the first cavity. Each cavity includes a bar pivotally attached to the block, which includes a flat surface for pressing normally to the bottom of the V-groove, along a vertical line of contact with the vertical cylindrical portion of a tool in the V-groove when the bar is positioned horizontally across the V-groove and is drawn to the block.