Hi Jim
John is right, Sounds like it's the Parking Brake Warning Signal, Buick Gr.4.591 Part #980876 I have one here that's NOS in the box . I found it in the trunk of a 65 Skyklark parts car that I Picked up in June. I never saw one before this one either. The writing on the box states that it's for 65-66 A bodies. we learn something new every day !!!!
Tony