Does the light fixture mimic the caps around the door handles and window winders? Do your window winders have metal turn pieces on the ends of the handles or are they a bakelite material?
I would assume that the piece that you are looking for is a bakelite material (probably a light beige colour) Unless the escutcheons and the overhead light match, I would thing that you could replace the overhead assembly with a repro. Our Frontenac, (similar in body style to the 619) has a switch on the B post and also on the overhead assy. (Queer) Let me know if all these pieces match. I believe I have and old window winder with the bakelite knob.
Where Are You From? Guelph, Ontario, Canada
Do You own a car built by Durant? 1932 Frontenac made by Dominion Motors Ltd. Leaside, Ontario, Canada