Transformer bell, and Choke housings need to have an electrical contact to the screws. One of the screws for each (rear most on chokes, and rear right on main transformer) are joined together on the under side by a wire to earth. Its for safety, and also to minimize any hum being picked up.
Any terminal standoffs that have connections on the middle post need to have a good electrical contact to the chassis plate as they are also part of the earthing circuit.