Semi-gloss black for the radio opening cover-plate (same as used on the steering column and steering wheel).
You most likely know that there are
black-painted "stripes" in the ribs on both sides of the radio opening? Including a few black stripes on the front of the ash tray. These black-painted stripes match the spacing of the openings for the speaker.
You can see the black stripes on the left of this radio opening.
I am not sure if anything was put behind the speaker openings, if there was no radio. Nothing is shown in that illustration above at group 16.083 (from the 1954 Master Parts Book).
