This is a pretty old thread now, but I designed and build the boards that Tom Hall and I have installed in Tiger, Alpine and MG tachs as well as some more offbeat applications. The board was initially developed to let me directly connect the tach output from a MSD 6AL box, but later I added an amplifier to allow the stock current trigger (common to all Smiths/Jaeger RVI tachs) to be used. As a bonus, that amplifier permitted the use of Pertronix and other points-replacement distributors without having to do any special tweaks.
Since 2001, I've built several hundred of these modules and Tom and I, as well as some other Tiger people with the requisite skills have refurbished many tachs, as an alternative to the work done by Nissonger, Mo-Ma (?) and to the newer stepper motor based tachs that Jim offers . Over the last couple of years, the demand has abated pretty significantly and it would help us to know, from Tiger owners like you, whether it's worth doing another run of boards.