4.00mm Popular Progress, high quality Carriage Clock Key. Manufactured from solid Brass, with a deep winding square and thicker stronger handle. Ideal for French clocks, Carriage clocks, & Wall Clocks and is fitted with an adjuster pipe of 2.00mm on this size of key. All of the key is manufactured in our workshops in Derbyshire, England.
The size listed refers to the distance between the flat section inside the winding square.