Tachometer help

[1973mutt]

I have a msd 6a and am having trouble figuring out a tach. Is the white wire the signal wire? Do I need a tach adapter? Trying to hook up an Amazon special and can't get it to work any advice appreciated.
New to me 73 Challenger 383 4 speed

 

[Challenger RTA]

TACHOMETERS The MSD Ignition features a Gray Tach Output wire that provides a trigger signal for tachometers, a shift light or other add-on rpm activated devices. The Tach Output wire produces a 12 volt square wave signal with a 20% duty cycle. Some vehicles with factory tachometers may require a Tach Adapter to operate with the MSD. For more information on Tachometers and MSD Tach Adapters, see the Tachometer Section on page 6.If your GM vehicle has an in-line filter it may cause the tach to drop to zero on acceleration. If this occurs, bypass the filter

White
This wire is used to connect to breaker points, electronic ignition amplifier output or to the Yellow wire of an MSD Timing Accessory. When this wire is used, the Magnetic Pickup connector is not used.