1974 Dart Sport 489 Differential Question

Hello, my Hang 10 purchased used came with the dealer sticker and originally was a 318 with a 3.23 axle ratio. It now has a 360 and a 8.75 489 differential case. According to my factory manual, 1974 Darts did not have the 8.75 available so my question is:

Is my 8.75 489 case a retrofit or were there some 1974's supplied with the 8.75 489 from the factory?

Thanks.