These come up on ebay frequently...
http://cgi.ebay.com/ebaymotors/Mercedes-W114-280-Instrument-cluster-Speedometer-L-K_W0QQcmdZViewItemQQ_trksidZp3286Q2em20Q2el1116QQitemZ160212526022QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ 5fAccessories
http://cgi.ebay.com/ebaymotors/MERCEDES-BENZ-W108-W109-SPEEDOMETER-300SEL-280SE_W0QQcmdZViewItemQQ_trksidZp3286Q2em20Q2el1116QQitemZ200285123258QQptZMotorsQ5fCarQ5fTruckQ5fPa rtsQ5fAccessories
http://cgi.ebay.com/ebaymotors/Mercedes-w108-VDO-Instrument-cluster-1085425301_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1205Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318Q7c301Q3a1Q 7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem260335732740QQitemZ260335732740QQptZMotorsQ 5fCarQ5fTruckQ5fPartsQ5fAccessories

I don't think that the prices are reasonable but you need to broaden your search, it was used on more than just the 108...
btw, I have never seen one with the 6th digit.

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