I think that truck originally may have had a radio. I also think those tabs fit through slots in a fiberboard plate which held the speaker. Our task is to figure out how to attach a speaker plate either in a different fashion or to utilize the tabs which may or may not be in place...

Thanks very much for the image.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end