Where does that RPM information come from ?
 

I have a 1967 Marinette with a Chrysler 318 and i was looking at the engine
the other day, trying to figure how could my dashboard display the RPM from
the engine. I found nothing coming from the engine to the dashboard except
the coil wire. To my surprise i found under the dashboard a little grey
box to which that coil wire came in then came out with different wire color
to the tachometer !

What's that box ? Is that 1967 technology or what ? Does that box convert
a signal from the coil wire to interpret RPM ?

Any clue ?