Correct, 1950 as shown below.
https://www.flickr.com/photos/advance-design_parts_co/8251910066/in/album-72157632186114015/
Another nugget from the 1948 Full-line brochures:
https://www.flickr.com/photos/advance-design_parts_co/8131119337/in/album-72157631872034068/
https://www.flickr.com/photos/advance-design_parts_co/31521030230/in/album-72157631872034068/
Option No. 390 in 1947 and '48 books is same as below.
Option No. 390 in the 1951 Salesman Data Book, so it made it through the widened seat change.
https://www.flickr.com/photos/advance-design_parts_co/31777683041/in/album-72157631872034068/
My 1952 book makes no mention of the Deluxe Cab option, I am guessing because wartime chrome restrictions made it a non-option. Corner Windows were available as Option No. 387, but no mention is made of the seat back cover.
No 1953 book, but my 1954/1955 books, Options Nos. 430/431 make no mention of the panel. So my conclusion would be this panel is a fixture of 1947-1951, Deluxe optioned cab trucks.