This is the left side barrel screw also can be called shoulder screw that fixes the cover to the mech. Fits R thought DS

NOTE:  on some mechanisms the bracket this screws into is closer to the cam, before installing check clearance on mech end cam as you may have to either backout or shorten the length.  This has been noted on the 201 and 161.  Also note if you have a new rubber grommet installed it may be a tight fit, slightly oiling the barrel will allow it to install without binding on the rubber.