Mavic Simplex Downtube Shifters

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I need to replace repair rebuild my rear derailer shifter. It is a Mavic Simplex shifter it says RD 12.9 on the screw that attaches it to the frame. These are on a 8 speed Kestrel with a Mavic group. The shifter will not hold the gear. There is no way to tighten the shifter so that it will stay in gear.

[Bill]

Would you happen to have a picture?