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Gentlemen, thanks! This helps end the mystery.
Warren, I found a Bosch intermediate unit but I can't bring myself to cut it open for testing! But knowing what the early tachs want for a signal makes it easier to come up with something. I do find the inductor approach too inelegant, however, so I am planning on doing one or both of the following:
1. Fab a transistor buffer stage out of a single transistor, with 24V from a 12V-24V DC/DC converter to the tach /1 terminal and the collector and the current from the CDI's points trigger to the base, emitter to ground; or
2. Finish my LM2917 tach project (I was missing a few of the supporting components but I can get them from Radio Shack) and install the new circuit on prototype board in an old '66 912 Tach I have (I'll have to either have a new redline screened on it or, more interestingly, scan and reproduce my own gauge face using a color laser printer).
Other than those alternatives I don't think there are alternatives to a big, heat-producing inductor.
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