not paying attention when you put the battery in backwards, you welded the cutout contacts int he 40$ vreg together.. vreg is toast .. genny probably didn't like it at first either.
now at shutdown.. the battery discharges thru ammeter into generator as it is trying to be an electric motor.
genny is probabbly really hating continually draing that battery down.
I bet battery don't appreciate being used int hat manner either. you have probably shortened it's ultimate service lifespan significantly.
alng the way.. the wire path these moving electrons have taken are thru the ammeter.. a shunt device.. that's probably gotten pretty warm.. pretty much too often. plus the associated wireing that may have suffered from constant high drain of the shorted cutout and non turning genny (motor ), stalled, max amps.
pull line from genny bat lug, clean points nad regap..
charge that poor battery up again.. and hot wire coil from battery to bypass the brutally toasted wiring. if she starts up.. you are half way there.
hookup wire back to vreg... if ammeter shows a charge you are one luck person, as the genny itself survived.
as a test.. shut down and notice ammeter swings negative. disconnecting bat lug on vreg should 0 it again. if so.
might make some NOTES on the vreg can or side of the battery on how to hookt he thing up to prevent this type of thing from happening again. On the other hand. I doubt you will ever forget it now.