I don't know what that screw is for. My choke bridge just slides over the end of the choke control shaft. The parts diagram does show that screw, with a special spring washer and nut, positioned to go through the slotted end, so as to clamp it to the choke control. Mine never had one, so I guess it's not really needed.
The earlier models up to the K3 had that bolt and a nut, which is missing. It went through the the bridge passing just above the butterfly shaft and was intended to fasten the bridge to the the shaft. It proved to be unnecessary and was omitted from K3 on.
Leave it off.
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