This looks to be an adapter that converts from a 12V pulse to a signal similar to that from a conventional coil/ignition. I don't know if this particular product will work with the 914 stock tach however. The old VDO tach is somewhat finicky with what it wants to work correctly. I have seen some folks posting having trouble with their tachs with only their points being worn or improperly adjusted.
http://race-mart.storesecured.com/items/products/auto-meter/AUM9117-detail.htm