[CompClassics]

It is a Smiths Chronometric and utilizes a mechanical drive. There are various models of this tachometer for different applications and where the tachometer drive mechanism is attached to, crankshaft, distributor, camshaft, etc. the drive SAI used was off of the crankshaft with a specially fabricated bracket and gear box, the ratio would have been a 2:1 ratio. There are also different variations of the tach that would have been competition specific with the red needle that would indicate the highest rpm achieved, it also has a reset button for the tattletale needle.
I have this setup on my Cobra, we fabricate the bracket and drive assemblies per the original SAI design..
The tachs have been increasingly hard to come by and are fairly expensive when found. Make sure the tach. You get fits the application, Nisonger can do repairs and ratio conversions also.
If you would like more info email me directly at jkb5 at sbcglobal.net
The correct tachometer with the tattletale feature is an RC83. EX2