Greg,
   The plastic pin is the limit to the calibration slipage...the rubber bushing behind the dial skirt keeps the shaft and knob together. If the rubber bushing is missing, worn, or there isnt enough tension against it when puting the knob and skirt on, there will be slipage and loss of calibration. The plastic pin keeps you within a range of adjustment and keeps the dials and PTO from getting too far out of cal.
 
73, Gary

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