Wow. Excellent info.
I run previous generation with no labels or docs.. and learned info leaving some in disebelief. a lot of the diffs can swap out even today, right back to the old subes... if you have an old sube, I can sincerely answer some questions (the ea82 generation).
I have had most all models, including the "rare" 2wd 5 speed. (The 1980s xt has even more complex things I have yet to play with...)
As it turns out.. this lack of labeling only reveals subaru has had all that is written today in different designs, all mechanical... and add two more versions:
the dual range,
and full diff lock for rear (that is the absolute einstein of subaru history to me..stil not replicated)
the RX has an extremely rare 3.54 diff (top speed with 13 inch wheels has a 160mph potential)
the 3.9 diff was everywhere..(and the 3.9 had several einstein versions.)
and the the 3.7 diff is an oddity I have yet to find written correctly. (by chance, for example, I had an 87 DL with a 3.7... I needed a rear diff, and could not even find one.)