2 wire alternator

A 66 would have a single field wire connection and one connection for the output. 70 and newer alternators had two field windings and an output connection.

The 70 up design is a better performing alternator and eliminates most of the dimming of the lights at idle the old style experienced.

If some one installed the 70 up style it also would require the new style alternator that uses the triangular plug.

One of the field windings attaches to the field termainl on the regulator, ususally green. The other field winding would be attached to switched 12v, ussually blue.