I've since also learned this about those early gauges:

What you have there is a "balanced magnet" gauge (instant response).  They run on full battery voltage.  In theory they work on opposed electromagnetic fields and the actual operating voltage won't affect them as much as the later gauges.  I found they can be calibrated using a good sending unit on the bench and moving the two slotted nuts on the back (these move the coils).  It is ABSOLUTELY IMPERATIVE that the balanced magnet gauges have a clean ground connection if you want them to read  correctly.  This system was also used by GM at some point and there are a few articles on the net about how they work.
If you don't have a good sending unit (and these used different senders than the later bimetallic gauges) you can fake it.  A 10 ohm resistor in place of the sender can duplicate "empty" and a 90 ohm resistor can be used to simulate "full".  This is exactly opposite of the later senders for bimetallic gauges which provide about 270 ohms empty and near zero ohms for full.

   "Boink" <
   When I owned my first Mark I Mini (a '62 850) I had problems with that tiny fuel gauge inside the speedometer assembly. It was inaccurate at the low and high ends (though only the low end concerned me). I finally built up the courage to remove this antique gauge in the center pod and opened it up (by, as I recall, gently prying off the metal cover over the fuel gauge section. Inside, there are 2 electromagnets that can be very gingerly repositioned (but one must be very careful with some extremely delicate wires). It worked and I ended up with an accurate fuel gauge! I hasten to add, though, that all this presumes a good, working sender unit (since all this is a function of voltages, and the sender is basically a variable resister). Anyway, in the process I drew up a little wiring diagram that shows this, which I just found the other day... and if you'd like it, go to this web address and just copy it. It's on me.  http://www.aracnet.com/~kaminski/fuel%20gauge.htm

   - MARK