The diagram is more difficult but you can spot whether it has the optional setup pretty easily.
On the firewall you should see the grommet with two heavy wires coming through.
Post some photos and we can point it out if its there. We might see other things too
H'm post some photos of the engine bay firewall, battery and alternator and voltage regulator.
If the ammeter was pointed 1/2 way toward charging, that would be 20 amps.
That's not a problem for a few minutes, but if it continues for long time at 20 or even 10 amps, that indicates a problem.
When its doing that, put a voltmeter across the battery, and if you see 18 Volts you like you did, then problem is probably the regulator.
Confirm its the regulator by measure votlage to ground by probing the connectors closest to the regulator's 'sensing' wire.
On '74 these would be the blue wire feeding the alternator's rotor and connector at the ignition ballast with blue wires.
I can post photos later.