My first Type 14 Nambu

[jmoore]

The square hole is the recess for the locking block which allows it to swing away from the bolt locking recesses during recoil.

You have the firing pin, it just needs the tip replaced. Pretty common repair, actually.
If you are careful, you don't even need a lathe to do a good job. A short length of drill bit shank can serve as the new tip.