A WORM SHAFT (21) DISPOSED ON AN UPPER SIDE OF A MASTER GEAR SHAFT (8). A SPOOL SHAFT (16) DISPOSED ON AN OPPOSITE LOWER SIDE OF THE MASTER GEAR SHAFT (8). AN ENGAGING MEMBER (22A) ARRANGED TO ENGAGE A SPIRAL GROOVE (21A) IN WORM SHAFT IS ALSO DISPOSED ON THE UPPER SIDE OF THE MASTER GEAR SHAFT (8). A SLIDER BODY (22B) INCLUDES A FIRST MOUNTING PORTION (22C) DISPOSED ON THE OPPOSITE LOWER SIDE OF THE MASTER GEAR SHAFT (8) AND IS A PORTION TO WHICH THE SPOOL SHAFT (16) IS FIXED, A SECOND MOUNTING PORTION (22D) THAT IS DISPOSED ON THE UPPER SIDE OF THE MASTER GEAR SHAFT (8) AND IS A PORTION TO WHICH THE ENGAGING MEMBER (22A) IS ENGAGED, AND A COUPLING PORTION (22E) FOR COUPLING THE FIRST MOUNTING PORTION (22C) AND THE SECOND MOUNTING PORTION (22D) SO THAT THE MASTER GEAR SHAFT (8) IS STRADDLED BY THE FIRST MOUNTING PORTION AND THE SECOND MOUNTING PORTION WHEN THE SPOOL SHAFT (16) IS DISPOSED MOST FORWARD. (FIG. 4)
