actually TO ME it seems you got something grounded to chassis, not really ammeter, alt or reg fail. Probably some of the MAIN wiring system ( wires linked to ammeter )

Ammeter is a gauge ALLMOST IMPOSIBLE TO BEAT!!! is simply a hard sheetmetal jumper between both studs attached, where a magnetic charged piece swings out just in front of and needle is attached to it.

the only "fail" I can think of on ammeter and is easy to fix, is the stud becoming kinda loosen from ammeter jumper plate. This makes to swing the ammeter studs and gauge itself untill make the isolation to be broken. The studs are pressed into the ammeter jumper plate.

you could get a melted wire around which made chassis contact and that fried the fuse link. IT CAN BE isolations to ammeter studs too as mentioned ( stressed by the heat up to get cracked ), or ammeter wiring cover melted.

why is this caused ?... poor charging qualities on stock alts at iddle, which when reving up, causes lot of load going through on just one moment to recharge the batt. Now think on this happening from years ago... Needle going back and forth unnecesarilly

several solutions to this depending on what you want to do. ( I don't like MAD Electrical solution as many of you know LOL )

running out of time to explain each one ( have to go to job ), but lot of ppl here knows what I will tell LOL, and sure will post too.