That's not really a *bad* price, when you consider the cost new....
The gear and quadrant arrangement is for compound / differential dividing as well as spiral milling - spiral milling useful in itself for multi-start worms etc (if your mill has a suitable gear to drive it from the x screw), compound indexing is Serious Stuff - the gearing can be set up to rotate the index plate as the spindle rotates ( but much slower!) and that allows.all the.prime-number divisions to be made.... There are a few pages of.Machinery's Handbook devoted to it. I've done.it once, to cut a 127 tooth gear for metric conversion, but under instruction!