A moveable tailpiece for use in association with a slip form on a curb forming machine. The two optical waveguides are arranged such that they respectively receive a measurement light portion of the measurement light and that the measurement light portions are superimposed after passing through the optical waveguides. The device has a capability of storing a coating medium. The main body is desgined also to thermally protect the element during part of the flight of the projectile, and to be soft enough when released from the projectile, so that the main body does not harm the projectile after it is released.