A fuse is a protection device, nothing more. To an extent, it doesn’t exist as part of the circuit. Its purpose is as a controlled point of failure if something goes wrong elsewhere.
It being unmarked, there’s no way to know what its precise characteristics are. Even destructive testing wouldn’t let you identify it conclusively.
I think you’d be better off learning about fuses independently of this, and for the purposes of this circuit, just pretend it’s not there.