Brass Clock Key Mainspring Winding Sz 10-13 4.75-5.5mm. This is a new set of 4 single end clock keys. They are used to wind clock mainsprings. You receive: One size 10 clock key with a 4.75 mm hole. One size 11 clock key with a 5.00 mm hole. One size 12 clock key with a 5.25 mm hole. One size 13 clock key with a 5.50 mm hole. 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.