In START position, the GRD pin connects to the ignition switch body. The wire on the GRD pin connects to the engine overtemperature lamp. The GRD circuit tests the overtemperature lamp when starting the car.
The GRD pin and wiring is not needed on the Riviera. The Riviera is the only '66 Buick using a 4-pin ignition switch. It uses a temperature gauge instead of a lamp.
The other contacts operate the same for both switch types. See attached diagrams from the shop manual for more info.