The description of Texas license plates for 1955 is correct: yellow with black characters with the two letters stacked on the left side.
Suspect Seth Roberts initially drives a stolen "Falldale County" vehicle which is a light colored 1955 Ford Customline Fordor 4 door Sedan. After the robbery, he switches to his dark colored 1955 Mercury Monterey 4 door Sedan. Dan Mathews is driving a light colored 1955 Plymouth Savoy 4 door Sedan.