Abstract: A dry wall interlocking building block system includes a method of manufacturing moulds for casting building blocks, in which master forms corresponding in shape and size to the required blocks are cast from a cold-pouring compound in collapsible core-boxes, and in which a mould is built up around the master forms (7) by securing the forms to a casting table (8), placing metal liner plates (9) against the sides of the forms, filling the spaces around the liner plates with a similar cold-pouring compound, allowing the compound to set, and withdrawing the forms to leave a plurality of mould cavities. The mould can then be mounted in a block making machine for large scale casting of concrete blocks in the mould cavities. One feature of the preferred mould is that the liner plates are detachable, countersunk bolts (10) having been fitted in holes in the liner plates (9) prior to filling the spaces around the plates. The invention also includes a two-recess building block cast in the above mould.