They are interchangeable as I just put a late 2.5 CR StR tailgate on my Series 1 D22.
The differences are that the later one is thinner and rounded at the top, and the early series has a welded in bolt in the bracket, the later uses bolts.
You probably need both the tailgate AND the brackets as the bracket lengths vary between generations.
Without having a later Si, an Sii, Siii to compare I cannot tell you at what point the brackets change.