Image Item No. Description  32324  Technic, Brick 4 x 4 Open Center Catalog: Parts: Technic, Brick  32531  Technic, Brick 4 x 6 Open Center Catalog: Parts: Technic, Brick  32531c01  Technic, Brick 4 x 6 Open Center with 2 Fixed Rotatable Friction Pins on End Catalog: Parts: Technic, Brick  32532  Technic, Brick 6 x 8 Open Center Catalog: Parts: Technic, Brick  32532c01  Technic, Brick 6 x 8 Open Center with 2 Fixed Rotatable Friction Pins on 3 Sides Catalog: Parts: Technic, Brick