We can beat on California for a million and one things, the poor imigration policies for instance. But you can't beat up California on this one, it has nothing to do with laws, it was supply as I had said. There was also a different one piece frame that was used for the same reason. And Canada used those one piece bumpers too.
From a web site I found with a quick google.
I've been curious about the seamless frames and one piece bumpers for a long time, and here's what little info I've learned: Not all CA built cars had seamless frames; in fact, most had the 2-piece frames. According to info from Bob Wingate, a 55–57 guru I talked to 30 years ago, GM did not produce the frames in-house. They had two outside companies produce them for Chevrolet. One producer was A.O.Smith, and they produced the 2 -piece frame. I don't know who the other producer was, but they made smaller numbers, and they built the seamless (also called California, West Coast, Nascar, or tube) frames. Gm had to ship the frames from back East plants to all assembly plants, and they were charged by weight, so they tended to ship the lighter seamless frames out West (furthest), thus more seamless frames ended up under West coast cars than others. But by no means were seamless frames confined to West Coast cars…they were used under all models except convertibles, and used at all assembly plants. Incidentally, I've seen seamless frames under Chevys clear up to 1964, so Chevy continued the practice after 55–57. The one piece bumpers were used on all CA-built cars, and I've never been able to learn why. It was probably an issue of having two suppliers of bumpers, with a smaller stamping plant only having enough capacity to supply the two West Coast assembly plants. We have to remember…GM was not producing collector or special interest cars back then…they were only building production cars, and they used parts from various suppliers as long as those parts met their specs. I hope this sheds so light, and I also hope it causes someone with more info to respond and give us the real poop on these curiosities.
Here is the rest of the thread, pretty interesting stuff. http://www.new.carsinc.com/mr.bowtie/16248
I looked at a number of sites talking about this and could find nothing on any California law. I still see these sort of things today, I have a cousin in Detroit who has worked for a number of different manufacturers of parts his whole life, it is common place to change suppliers and thus change designs. In that 41 Ford case I have no idea it could very well have been standard and deluxe. But without any more imput manufacturer/supply could just as well be the reason.