DIS differences

I spent 2 days calling around trying to figure out why my 91 sc has a dis from a 89-90 ford said that dis wont work correctly on my 91 so i started looking for one for a 91 but here in Tucson AZ there is a 150 dollar difference from the 89-90 dis and the 91-93 dis. If there is no difference why the cost diff and the part # diff?

This question has been asked before, but the answers have been inconclusive. The 89-90 DIS will work in the 91-93 cars, however, with no apparent ill-effects.