Dave;
Up to chassis 10,000, Gen 1 Tachs were fitted, so that's what your car would have had originally, but unless you bought the car new, you can't hope to know what's in there now for certain...and it's a similar situation with the Engine power...that was originally determined by (mostly) Cam, Manifolding, Carburretion, Compression Ratio, and Ign Timing Curve, so who knows what it might be today...if you absolutely must know, some measurements like Camlift and timing, and C-R would have to be performed...
...getting back to the Tach, it's no surprise to me that what's in there now, a mongrel oddity really, doesn't work..it looks like someone made a hybrid of Gen1 internals in a Gen2 Housing as strictly a Dashboard holefiller (and few besides Mark Olson and me would probably immediately notice and recognize this!)...I predict it has only the meter movement, and no internal electronics whatsoever (but this is possibly a benefit, making it a candidate for an electronic conversion! **)...I further expect if you were to remove it from the Dashboard and inspect the rolled edge holding it together, you would see it has been "unrolled" to open and turn into the Frankentach it is now.
See also:
http://www.sw-em.com/Smith's%20Tachometer.htm (I guess you already did, because that's where the first two pictures came from...so this is for anyone interested and reading along...)
Good Hunting!
** I might be able to supply contact info of an electronic specialist in Germany who has designed and built a modern electronic drive for a Gen1 Tach for someone there. Contact me directly to discuss. ( I have no financial interest or arrangement with him, but would accept a case of Guiness Stout as a brokerage fee if it worked out...!)