Here is information on what a Ruckstell is, and why important, from the guy that wrote the book on Grover Ruckstell.http://www.chaffinsgarage.com/l-z/ruckstell.htm
With a standard T gearing, you have low and high. A Ruckstell 2 speed rear axle turns a 2 speed car into a 4 speed, greatly helping with issues in the lower parts of the gearing range.
It was a $69 option on a $400 car. Can you name a modern option that would raise the base sticker price by that percentage??!
Here is a cut away of the internals of a Ruckstell car rear end. It is a planetary system, achieving a 50% gear reduction over the standard rear end. In Ford Standard (Ruckstell Hi=Fo), the gearing is identical to a stock T. In Low, a Sun Gear engages three pinion gears to achieve the reduction. Therefore your gearing is RuLo - FoLo - RuHi - FoHi
