The positive stops are typically bolts for the forward/backward. It does prevent moving the shifter a certain amount. It is not stiffer springs.
Here's a 'factory' Hurst unit. I think these were used on some of the early SVO and Cobras. Hurst offered them for a while too and then changed the design. This should give you an idea of the concept:
http://www.ebay.com/itm/5-0-Mustang-...r#ht_558wt_932
Here's a standard $50 shifter. The stops are not installed in the picture, but the hardware packet has the bolts:
http://www.ebay.com/itm/83-04-FORD-M...ht_3141wt_1018
Here's what that $50 shifter will look like with the bolts installed (this is a more expensive shifter):
http://www.ebay.com/itm/E-HP-Mustang...#ht_4491wt_821
And here's the $30 shifter:
http://www.ebay.com/itm/01-02-03-FOR...ht_2994wt_1160
Same thing as the $50 shifter.
