The L stands for lamp. The above poster has put this in for you.Positive is fed from the battery through a lamp on the dash down to the L terminal which in fact is a Diode. When the alternator starts to turn the current flowing through this Diode circuit excites the field and the built in regulator takes over.
The + terminal is simply conected into the Battery Positive wire somewhere. This feeds the charge to the battery.
Best type of Alternator to have in my opinion.