74 pantera tachometer electrical polarity?

The Pantera tach typically doesn't work with most electronic igntions unless an isolator is used. I'm told that the OEM tach circuitry is a fairly crude counting circuit as such things go, and will not be damaged by wiring it up wrong. More likely, it just won't work. MSD is aware of the problem and sells two different isolators as well as requiring a reverse-current diode to allow you to switch the ignition off; perhaps Mallory also has such things. Try calling their tech service dept in Sparks, NV for specifics. Sorry I can't be of more help.