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That's it, just blocking a machining access hole.

Bob
tufcj

The ball and spring are under the shift rail, you'd have to pull the shift rail out to get at them. It's just a machined hole so you can get them in. You don't want anything to thread deep enough...

I'm pretty sure the switch is a 4WD indicator light. The other is a plug, looks like the plastic cap is gone. It just screws in. Screw it out, clean well, fill the top (the end toward your finger)...