Follow the current out of the alternator and look for where voltage could be dropping.
Assuming this is an early system...
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Since the ignition feed is at 14.1 V, and the battery is at 13.5 V, my guess is the ammeter is showing a slight charge when you took those measurements. If so, modify the above drawing to show some current flowing to the battery.
If it is an early system, measuring voltage after the regulator is not as useful as measuring at the regulator.
Unlike the type shown in post #9, the earlier positive controlling regulator will reduce voltage into the rotor windings.