If it were intended that anything was to be screwed flat to it, it would be machined flat instead of left as cast.
No parts list views or indexes show anything as missing from there.
Pictures instead of the real thing, slight disadvantage,,,,,,,,,,,,,,,,,,
It still looks to me that the end of the forward shift rod is directly behind that Threaded hole?? ??
If the threaded hole were to be perfectly over the end of the forward shift rod?? it would be the perfect place to wind in a "jacking screw" as a facility to separate the center cases.
It would be nice to have closure, but I ouldn't worry too much about it Hardo,,
Do what you do best
fuel it up and ring its neck