Those parts were part of the radio (which was an option when your car was built). They were frequently tossed away when the owner bought a Emerson, Pioneer, Seiko, Hitachi, or other import radio to get a stereo FM cassette/eight track functions (carving holes to mount the speakers in the deck lid and inside car door skins to mount them). This is because the Jap radios used a different mounting kit with different knobs that wouldn't interchange with the original parts. Speakers were different OHM rating as well, in case you try and use them with a stock radio.
If you find a replacement Delco radio they should have those parts on the front when you buy it. It wasn't until the mid to late sixties that FM was even broadcast nation wide so a Delco AM-FM radio was rare and expensive.