2 speed aux gear box

[jqwiii]

I have a 9N that has been in the family since it was 1 year old.  It has an aux gear box with the shift knob mounted below the dash that gives a high and low range to all the transmission gears.  That makes it 6 speeds forward and 2 reverse.  It can be shifted from Low to high or High to Low on the fly by clutching.  Can anyone tell me more about this feature?  I'd like to know what this feature is called, any details available, how it affects the value of the tractor, etc.

[jeep]

http://home.att.net/~jmsmith45/sherman.htm           

[lha]

Thats a good old sherman od tranny,got one on mine,really handy when on street for short runs.Third-high will give you about 20 mph,as fast as you want to go!---lha