Hi...sorry I've been up in Arkansas for a funeral. Actually I'm saying I believe that piece marked by the green arrow has become over-magnetized by too much voltage/current. Which can happen. A strong EMF can produce a magnet or it can raise the strength of an existing magnet if the existing magnet is oriented correctly. That piece is supposed to sit as you see it there, but a little further over to the right. The magnetic piece in question is sort of hourglass shaped and the needle comes off it at that angle. It isn't the sudden higher voltage causing the problem. 12v is 2x the 6v to 7.5v the gauge was designed to operate with. And I believe the reason the gauge (which is in otherwise good nick) won't work with either 6v or 7.5v is because that hourglass shaped piece of metal (which was always magnetized) now has been overmagnetized. When 12v is applied to that gauge, the needle will travel right to the F mark crisply. Remove 12v and apply 6v or 7.5v or even 7.75v and the needle will not travel up to the F mark at all. But if you apply 8.4v or 8.5v, then the needle works correctly again.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end