@Matt;

I also knew and understood it the way Phil explains it...on the early cars, the tach circuit module with electronics was located in front of the radiator of all places...I expect they wanted to keep the (sensitive transistorized!!) circuitry out of the high temps of the engine compartment, but this separate module also used the inductively coupled sensing strategy, and I expect this was specified to assure that no failure mode of the tach could disable the ignition system...the tach being from Smiths (a British automotive electrical equipment supplier...and all that suggests!), someone thankfully had the forethought to specify this important reliability factor...

The difference in redline on the tach face is also a way to tell if it is an internal or external circuit type when buying used tachs.

Back to the original issue: In general, interfacing aftermarket electronic ignition modules which still use points as triggers to the OE tach present some unique problems. After installation of the module, the tach typically wont work because the points now only trigger the capacitive discharge and so do not pass anywhere near the magnitude of current which they did, when they controlled the coil primary current directly. This means that the points line cannot simply be wired in as it was because the pulses coupled into the T1 primary do not have a sufficient current/turns ratio. I'm thinking that a solution might be to increase current/turns ratio to by simply increasing the number of T1 primary turns (number tbd)...this would increase the pulse magnitude coupled in, to something of enough magnitude to allow tach to work...oscilloscope measurements would be necessary to verify this concept and to be able to calculate how many primary turn would be required...

Cheers