2-speed rear end – questions

I am looking at another '70 barracuda and this car has been upgraded from a 318 to a 426 w/727 and the seller said it has a 2-speed rear end. He has a switch by his seat he reaches down and changes performance level. Now, I understand the high/low for performance and hwy, but what is it? Factory? aftermarket? who made them? Worth $?
I've never heard about them on a cuda and don't know much about them. Are they rare? or good to have? or not?
Thanks for your time.
Author: hofilbert