If those Smiths tachs operate on the same method as the one in my (long gone) 73 MGB-GT, they are a current sensing design. The wire from the distributor points to the tach has a loop in it so that it makes 2 passes through a device on the back of the tach. When I installed a Crane electronic ignition in the car, their instructions advised that the distributor-to-tach wire may need to be "de-looped" so that it made only 1 pass through the back of the tach. This was to be done if tach readings were inaccurate after the conversion. Mine did not change.

Since there was no direct connection of the wire's conductor to pass electrical current or voltage into the tach, I assume that the tach's internals detected a magnetic field generated by the dizzy-to-coil wire (or ignition amplifier-to-coil). The strength of the field would vary with current flow, and its frequency with RPM.
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Bob: Son's XC70, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles