Good morning c\_/
That is the add on instrument voltage regulator that I designed, built, and sold for several years. Other gauge service companies would buy 30 to 50 units at a time from me so I can't know if your car was my customer. One of those companies bought me out, lock, stock and barrel. I retired. That doesn't mean I can't help diagnose and cure.
This aint the best place to do that. PM me if you want.