Scott, some of those Saginaw power steering boxes have a tall tooth on the center of the worm gear. It makes for a very quick response just off the center point, and the ratio slows down as you approach the end of the helix. I think the variable-rate design originally came out on Cadillacs, and pretty soon it was adopted all across the GM carlines. Not too sure about when they started putting them on trucks. It made for very quick lane changes while still allowing a good mechanical advantage for parking maneuvers.
Jerry
