Two halves

When we take a side view, it looks as if both pieces are parts of one single stone that has been split in two - and perhaps they are. The inside of both pieces is more or less flat, although not polished, and the outside is rounded.

This is not a random piece of stone with some text on it, but a carefully shaped object that covers and protects the information written on one half.

So what was the function of these stones?