RTFM?
The bolt you are pointing at locks the adjuster. The original idea was to have the bike running or set the crank at a special spot then loosen that bolt. The adjuster would self tighten itself to the correct tension, then you would tighten the lock bolt again.
The open hole above your finger is a place for a plug bolt. The springs that work the self adjuster and the actual adjuster rod are under it but nothing comes out through there.
After a few decades the springs lose tension and to actually adjust the chain you have to take out that upper plug bolt and push the adjuster rod down with a thin screwdriver or something, helping the tired old springs.
The bolt you say is broken is a complicated piece with two different threaded sections - 6mm for the actual screwing into the case and 8mm for the locknut. Usually it breaks because someone overtightened the locknut and the stub below can be removed by sawing a slot in it and using a screwdriver, if it was overtightened into the case that gets a lot less easy.
