M!ke, I've started my winter vacation doing the most possibly exciting thing (studying the Goose wiring...) and wrt to this question, I think you actually had the answer yourself on another post (Oil Pressure Sender Woes). I'd first thought this was the gauge dimmer too, but I think the mystery device here was intended to be a voltage regulator for the gauges. Interesting on the original (Italian) schematic, the positive terminal for the gauges (for those that have senders, the oil/temp/temp/fuel) is connected directly to the fuse position, but on the Hall schematic this mystery device is between the fuse box and the gauge Positive voltage reference for the gauges. British cars used something like a Lucas 37584, in Lotus Europa mounted on the back of the temp gauge.... Maserati Ghibli used a voltage stabilizer but the footnote seems to be only when with Smiths gauges https://www.maseratinet.com//p...abilizer-gauges.aspx
Without such a regulator, the gauges will almost certainly drift as the Alternator speeds up...So convenient to use, in case you like the comfort of lowering the engine temp by turning on the lights 
...But neither is it clear that anything was actually installed, the parts diagram for the Goose doesn't include any such device (well, neither does it include a switch for the parking brake, which shows up on both wiring diagrams but maybe not a single car...!). I would have expected such a device to be on the firewall next to the horn relays and blinker cans, but I never saw anything on '1076 (and you may just follow the path and convince yourself it wasn't ever there...). Lastly, from what I see from Pantera schematics http://www.panteraplace.com/page107.htm there was no such regulator for the Pantera either.
For certain, adding the voltage regulator will cause all the gauges to read lower (since the regulator will drop the voltage to the gauges...), so this may answer the question you had ) but do nothing to help you on the problem! 
).
Fun looking at the wiring diagram, I had never noticed that the Hall trace left off the interior fan switch ! Lee
ps. Note, see attachment...another connection missing on the Hall trace is thru the Alternator bulb; The alternator lamp is not grounded, one end shares the Battery voltage (coming from the fuse and feeding the gauges), using a pink wire (the same as used also for the gauge's 12v reference). The other end of the alternator lamp follows a white/black wire to the 'i' terminal of the voltage regulator--this is feedback for the voltage regulator to control the field current in the alternator...A great picture of this is Bill Taylors Pantera drawing, below are the Hall vs. DeTomaso diagrams. 
