Coil testing machine

[Kube]

Hi Guys,
Right off I must admit I know very little about your beloved Model T's. The fact that they are cool is nearly where my "knowledge" ends.
So, here's my request:

I have a coil tester, um, I believe that's what it is, and would like to know more about it.
Rarity? Desirability? And yes, value.

Thanks in advance.