Dave - That is what appears to be a chip missing in the hard rubber of the metering assembly. What you see in the triangular hole are the gears that work the speedo cable.
I'm not sure what to do right now, a new assembly costs about $60 which seems a little steep for something I don't need. (I'm using a sigma bicycle computer)
Anyone got a suggestion on how to fix this one, or got a used one for sale?