Can anyone identify my rear differential? 8 3/4 but wrong markings???

This is in a rare 1935 Australian Ford Ute I bought. I'm planning on making it fully Mopar-driven. The diff was previously connected to a powerglide/283 though.
10-Bolt, Front-Drop carrier pumpkin with no access in back.
Driveline is at center of pumpkin.
Looks similar to 8 3/4, but has unusual numbers on it.
391 is stamped under the filler bolt.
5 is stamped on lower left.
Screwed on plates with 2 and 10 are on upper left.
Is it not 8 3/4?

Any help is much appreciated.

Diff1.jpg

Diff2.jpg

Diff3.jpg

diff4.jpg

Diff5.jpg

Diff6.jpg