In order to allow the user to see which cam surface (one of surfaces 60 e 1???60 e 8) is arranged in the path of the stop projection 40 i (i.e., to adjust the desired depth setting) the left body part 110 has a window W (e.g., a through opening or slot 110 p) through which one can read the indicia (e.g., numbers or other desired indicia such as letters or marks) of the cam wheel 60.