I'm guessing if it was a firetruck, there would be much more obvious evidence. Beside, that's a VERY queer location for a tach. A pumper would have it either up front for a front-mount, (again, there would be an extended bumper or something giving a clue), OR on the side behind the cab door, with the REST of the pump controls. Whichever, it would also typically have a remote hand throttle located near the tach somewhere.
The switch certainly looks like a siren switch. Maybe this was a Civil Defense truck of some sort? Not sure what they would need an alt. tach. for . . .
I LOVE a good truck mystery!
-Michael
