Alternator only charging at high revs

Hi, I need a little help here. My alternator (2 year old Powermaster) has stopped charging properly. Normally at idle in Neutral the Alternator gauge is angled to the right, showing it's charging. Now it's in the middle at idle, and foot on the brake it's to the left showing discharge.
Once I get on the highway and the revs come up to approx 2,500 the needle moves to the right again.
I took the voltage regulator off and the points had a lot of carbon buildup and were hardly moving. I cleaned them up and they move freely - I thought I had the problem licked, but no, still the same issue.
Car has been parked for a week, so I started it up and put my multimeter on the battery. 12.5 volts at idle (with Alternator gauge showing discharge) and maybe 12.7 at 2,500 rpm, even though the Alternator gauge was showing it was charging.
Took it for a run up the highway, maybe 20 mins each way. Got home, battery at 14.5 volts at idle, and 14.6 at 2,500 rpm. I don't think it's a gauge issue because the lights were very dim at idle when I first noticed the issue a few weeks back, but I'm totally confused.
Battery seems strong, spins the starter no issues, so it must be charging at some time.
First photo shows points before clean.


After cleaning

Thanks but I was looking for more specific help.
The alternator is the correct one for the car, it's been on there for 2 years with no issues. It's an externally regulated unit as the car is 1966.

Any Chrysler isolated field, externally regulated, alternator can be easily made to work for a pre-’70 mechanical regulator application. For this situation as described, I would start with a full field test, if charging voltage comes up immediately at idle, it would indicate a regulator issue. You can get electronic versions of that regulator that would be more reliable.
If there is no change in idle charging voltage while full fielded, would indicate something has changed with the alternator output capacity, i.e. diode failure.