That’s far from original. It’s probably been modified to work by picking up a voltage pulse directly from the coil or ignition amplifier, as in the Series 3. This is sometimes done to S1 tachs if they are to be used with electronic ignition. I suspect you could figure out how to get it to work, but it would require additional wiring. The easiest thing would be to get a different tach. They are readily available. Most have a black bezel (70-73) but your chrome bezel (68-69) can be swapped to the replacement if you don’t get a chrome-bezeled one to begin with.
There are a bunch of them on the USA eBay right now; here’s what the back looks like:
