Thanks for the point.
Actually the source says the illutration was made by Maxine Regan according to a design by David E. Scott, so the latter should be credited.
That's it! But dated 1949, for expected production release 1951 (so my source..)
1949 is the date for the other illustration in your source
This Plaza Landau Coupe proposal made in 1951 definitely has the styling features of 1953 Plymouths.
