Thank you markeb01.
The bracket in his picture is the GM outer bracket #141L cast by Autolite.
This is a picture of the GM inner bracket #8205L cast by Autolite.
Thank's to member ( creep ) for the picture.
Take note of the stubbier interior control handle for GM Factory/Dealer installed spotlights as compared to the standard handle from Unity.
Full instructions from "1963 Chevrolet Custom Accessories Installation Manual" The spotlight is model CH for Chevrolet. Has ribs cast into the head.
The spotlight for GMC is the S-6 225 smooth head.