All 120y coupe's came out with the jap diff and brakes only.
The 120y sedan's from 1977-78 had the BorgWarner diff in them and they had PBR front brake calipers and booster aswell.
I own a 1974 and a 1977 120y coupe and they both run jap diffs and brakes, I also had a 1978 120y coupe and that also had the jap diff.
The 77-78 120y sedans with the BW diff also had stronger/bigger tailshaft uni's in them and the flange where it bolts to the diff is slightly different aswell.
Cheers, James.