On our 1985 4/4 the ammeter is wired through an additional "starter relay" not shown on your diagrams.
The wiring diagram in our car's handbook also shows this relay.
Might this help explain what is going on?
Surely wasn't your car originally fitted with a voltmeter ? not an ammeter.
It seems the early Lucas 11ac alternator with its complicated external voltage regulation was fitted to Morgan's until at least 1984 according to various wiring diagrams. These models seemed to continue using ammeters although Land Rover (from which we can often draw parallels) when they offered this alternator switched to voltmeters in the late 60's as shown above.
I think Jack's assumption the ammeter is wired "through" the starter relay is a red herring. The battery side ammeter terminal (brown wires) is just used as convenient connection point for main power to relay which then triggers starter solenoid. No current flows through ammeter when starter is operated therefore.
