I've seen a number of melted ammeters over the years. It just wasn't a very robust design during the 60's and 70's. I assume they fixed it at some point but I had a '74 Dodge truck with a melted ammeter. When they started to use the big 100 amp alternators the engineers got serious and upgraded the whole system. I think they eventually got really smart and eliminated the ammeter. Not sure when that was for sure but you don't see ammeters in cars anymore. Someone eventually figured out that shoving all the charging amps thru the firewall and back out again wasn't the brightest idea.

Lots of those older ammeters used a stud w/ 2 nuts for the lug. If those 2 nut weren't tightened properly, you'd have a poor connection between the studs & brass plate of ammeter. That stud needed to be tight against the brass, then tighten the 2nd nut for the lug. If not you'd have a good connection between the 2 nuts & lug, but a Poor one between stud & brass.