Need identification help with a tach
 

I bought a car the other day and in the back seat was a stewart warner tach, it is very old and i have been told by a couple people that it came from a Yenko car. How can i tell if it is or isnt? I have a picture of it, i dont know if i can post it on here or not, if not i can send it to someone and if you could let me know, any help would be greatly appriciated.

Thanks in advance