What's the Little Black Box?

Under the cover are some slots numbered 1 thru 16 in white numbers.
Could someone please tell me what this is for? Ignition related?
There's nothing in the owner's manual I can find on it.
Also, a previous post mentions a cheap fuse "behind the battery" that the member said is supposed to improve starting/idling when replaced with a new one. I could not find one anywhere.
Thanks.


