Those are awesome tachs and pretty rare.. I picked up an NOS one off e-bay a couple years ago which was unopened still in the Chevy Box. I installed it in my 70 Nova SS. Some people think it is a fuzzbuster on the dash... You also gotta have the Chevy Bowtie Compass....
Very Cool Piece.... Seems like alot of money though for a used piece.. I paid $450.00 for mine as an NOS piece.. Just my .02....
Rich