Your meter could have been shorting out...making contact with ground. That would cause discharge. After reading your diagnosis, knowing your regulator is OK, I'm leaning towards your guage shorting.
See if there is continuity between each post on the ammeter and the housing. If there is, there's your answer.
~ Victor
1941 3/4-Ton Pickup (in process). Read about it
in the DITY Gallery1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~
moved on to fellow Bolter 
