That "tach" connector in the HEI cap was for the Handy Dandy GM diagnostic
console used in the dealerships. These were made/marketed by Kent-Moore to
authorized GM dealerships only.
Independent Garages had to make do with Snap-On, or other vendors
for those big expensive machines. I have used both kinds, and they all have
specific good and bad points. There are hand held devices today that do
much the same tasks on modern stuff. We use software programs for laptops
for the EFI stuff as well.
Jim Hupy
Salem, Oregon