I did find plastic material at a radio install shop that matches the grain almost exactly. I just made a plate yesterday to cover the hole left in my old cutout dash.The epoxy is now warming by the fireplace, I'm fixing it right after posting..
69 RA III T/A Auto 69 Sprint vert 3speed ( sold to a new home) 69 350 coupe ( new home in Denver) 69 350 HO 4spd
It shows 9796880 as 1969 all then 3903481 67 thru 68 and 69 all exc radio or stereo
I think that "10.252 Plate Instrument panel trim" is the instrument panel with the radio opening. It only shows different part numbers for AC/non AC and standard/custom trim (the woodgrain on custom interiors). No difference between radio/no radio which makes me believe that the instrument panels all have holes for the radio in them.
9796880 might very well be the radio delete plate (called "Plate, radio opening cover/accesory swith opening").
3903481 must be a cover to install instead of the speaker (called "Plate, Speaker opening cover").