power steering

I'm not really sure where to put this but i was asked this trivia question and its really bothering me now cuz i cant find out the answer.... the question is who was the first automobile manufacturer to offer power steering on it's entire line as standard equipment and in what year... any enlightenments??

In the 1920's through the 1940's, the development of a commercially viable power steering system was almost as elusive as the fully automatic transmission, something of a Holy Grail for auto makers.
Many designs were proposed, but I found the following brief comment off the net about the first viable design, which, as far as I know, General Motors introduced in North America in 1951, shortly followed by everyone else:

Francis W. Davis invented power steering. In the 1920s, Davis was the chief engineer of the truck division of the Pierce Arrow Motor Car Company, and he saw first hand how hard it was to steer heavy vehicles. Davis quit his job and rented a small engineering shop in Waltham, MA. He developed a hydraulic power steering system that led to power steering. Power steering became commercially available by 1951.

My note: Power steering was optional in many cars for decades and still appears on some basic models. My guess would be that Cadillac was the first brand to make PS standard in all its cars, likely in the early '50's