Oil restrictor?

[FJ1200W]

I noticed this earlier when playing in the garage -
Crankcases
O-ring's not going to work well there.
Thoughts? I'll see if it "pops out".
Center hole appears threaded

[FJ1200W]

I'm leaning towards it having to do with the starter drive being removed.....

I have a 1993 transmission on the way, making progress!