How is the voltage regulator functioning in this vintage charging circuit?
This SCR (not Triac) seems to shunt the core with one phase diode and reverse bias the other phase diode to reduce the generator output in order to regulate. The R divider biases the trigger threshold for 14.2V.

Not as mechanical load efficient as 3 SCR's but works. 

Lambda used TRIAC bridges in the old days (the '70's)  to pre-regulate DC before higher linear stages as a lab. power supplies.  That improved efficiency "somewatt" before SMPS came on the scene.