Several car companies explored making frames lighter and from lesser types of steel begining prior to WWII. Studebaker had a 'hat' type frame (cross section looked like a tophat), Nash used UniBody, and Kaiser used thinner guage metal with an X-member on the 51-55 cars. Well the idea was to reduce the use of specialized steel and as well as save material.
Mike Barker showed a 53 Traveler Kaiser at the Oshkosh National that had a boxed frame. That is, the side channels had panels welded on the inside.
Refering to your example, 3/8" seems rather thick and heavy to reinforce the bottom of the frame although it is easy to deform the bottom of the frame rails by jacking at the wrong point or running over debris in the road and you can do some repairs with just a 7" ViseGrips!!!