This is a small wooden box, usually cubical, 2-3 inches on a side. Usually carved or decorated in some fashion. There is a hinged lid on the top. Also included is a cloth armband, usually marked in some way to indicate that it goes with the box.
How it works is, you put on the armband. Then, when you open the box, you will see a smooth patch of skin that matches that of the wearer of the armband. Shots, as in medical injections, can be administered to the skin in the Shot Box, and the beneficial effects of the shot will be felt by the person wearing the armband, while all negative side effects are eliminated. As is the discomfort of the jab itself, obviously.
The Shot Box can also be used to draw blood for lab work or whatever. The nurse just sticks the Shot Box instead of your arm, allowing blood to be drawn without any discomfort whatsoever.
I know some of you will be curious. If you open the Shot Box when no-one is wearing the armband, it just appears to be an empty wooden box. This illusion will persist until such time as the Shot Box is opened while someone is wearing the armband. The “empty” box can be used to store the armband when not in use.