New Double End Clock Key Size 4. This is a new set of 2 size 4 & 000 double ended clock keys. These are used to wind clock mainsprings. You receive: One brass clock key with hole sizes 1.75 mm and 3.25 mm. One nickel plated clock key with hole sizes 1.75 mm and 3.25 mm We work hard to update our descriptions and pictures of our merchandise. As part of a process of ongoing innovation by our suppliers, newer versions of the item shown may be shipped before we are able to complete an update. As a result there may be a slight variation in color and design.