http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2413449190&...

stuffed if i know

i'm pretty sure that it isn't a tacho, but just a straight trip meter. ie for getting accurate distances when rally style navigating.
if it is like the later ones (with the odometer style readout) the adapter on the end of the cable is fitted inbetween the speedo cable and the mount on the back of the speedo. i gather the outside markings are 1/10 of a mile and the inside miles.
i have to say. that it is super cool and would be v.nice in a period rally preped split or oval.

[Edited on 7-5-2003 by whatnow]

Yep, that's what it does.You can mark a position in order to follow the map, or you can reset it for a trip. It was actually a good price for someone interested in that period.