Wire connecting alt posts?

It should not be marked (-) but I sure haven't seen them all. The important point is that both posts are 'hot' at all times. It must never be grounded. Some have the battery post marked 'R' or red because the battery feed is red insulated. The other post connects to the main splice (standard) or in very rare cases feeds back out through a grommet in the firewall to a terminal junction with the alternator and the bulkhead multi-connector.

That it works is not surprising because its not like the other gages:
The ammeter is directly in the battery output/charge line. Inside is a big metal plate and the needle is moved by small magnetic field created when current flows through it. (Except for 1976 models which have a remote shunt)
The other gages are thermo-electric. Those needles move as current flowing through creates heat that changes the bend on the hairpin shape. They need to be supplied through the instrument voltage limiter.

if you want help with the pin repairs, do a search and you should be able to find those posts pretty easily