Nothing particularly novel in all this but I've designed a jig which suited my purpose of mass producing blocks of varying width and constant height. The original technique came from Physicsman as used for his Fell walls on Kirkby Luneside 1/2.

The materials used were:

The base - a piece of laminated chipboard with a high quality relatively smooth finish. This is important because then the clay in the template mold doesn't stick and can be easily lifted out once cast but still wet.

The sides - strips of plastic venetian blind slats, 3mm thick, UPVC filled with calcium carbonate. These are tough and can be cut easily with a knife and planed dead square with a hand plane. As I was going to need different widths of clay strips, the second side was made easily moveable, held in place by two screws. 

The dimensions chosen suited my need, building a block veneer over a suitable former. Hence the 3mm was the depth of the veneer covering, the block width became the height of the blocks in the covering wall. The advantage over solid full depth blocks being several, quicker drying / more economic use of clay / lighter end product.