The Tudor Collection 613 38mm Cupboard Knob Smooth Black Iron has a round plate and knob making it a classic shape. The diameter is 38mm and the projection is 38mm - it is sold individually and comes complete with fixings. Smooth Black Iron Cupboard Knob The smooth black iron finish does not have deep surface indentations like many other black finishes - this gives it a more modern feel. It is made from malleable cast iron, treated with a zinc phosphating process and then powder coated to provide a durable finish.