John P has given the correct response. The slot in question is the black space in the tail stock in the picture. It is hollow. It just doesn't need filled and it also allows additional 'sound hole' function. It belongs there on a McSpadden. It is the way they are designed and built.
As for the base string buzz... try this: Carefully cut a 1/4 " inch square of black electricians tape and fold it in half, sticky side out. That's a small piece of tape and hard to handle, you may want to use a tweezer. Then loosen your base string just enough to slide the tape under the string, at the nut, not letting it extend past the nut toward the bridge. Retune the string. The tape will space it up enough to let you know if you have a high fret or a nut problem and then you can effect a more permanent repair.
Congrats on the purchase. I'm sure you will love your McSpadden.
