This is a horn relay with a battery junction stud.
Fits the vehicles below with a junction stud:
Buick Riviera – 1963-64
Buick Skylark – 1961-64
Oldsmobile Cutlass – 1961-67
Pontiac Bonneville, Catalina, Grand Prix – 1963-64
Pontiac GTO, LeMans, Tempest – 1964