I don't remember the specs, but the thrust block is smaller in earlier diffs...
I think it's about a quarter of an inch difference, it's a one-piece block that goes in every open carrier, it's also there in some of the later Borg-Warner cone-type Sure Grips, but not all. And, of course, the Dana Sure Grip uses the buttons.
The purpose of the buttons or the thrust blocks is to transfer the cornering load to the wheel bearing on the inside.