[Bob Ashlock]

Around August of the '71 production year, Porsche changed the tach to work directly from the points. Before that, the tach was triggered from a 'filtered' signal derived directly from the high voltage discharge A-lead of the CDI/coil. The 'filter' was the round-can "Intermediate Unit". Like Fred suggests, perhaps your tach was modified at some point, or it simply could be a later '71 version.