Obviously that is a good one... INdeed there may have been variations in the filter design/tach winding characteristics over the years.
In my world, since the signal fed to the tach is analog, I'd install a resistor to suit. I do not have values but measuring the tach resistance (disconnect tach at filter -mounted at ignition coil bracket - then measure between B white wire on tach side and ground) and then installing a resistor of twice the value in parallel (from B to ground) should down the reading close to normal.
www.avigex.ca/xport/instrumentpanel.jpg