It's called a slab. It's a board cut from near the outside of the log, when the saw starts to daylight out. I used to burn slabs in my woodstove, I'm very familiar with what they look like. Some of them are mostly bark on one side, some are sawcut on both sides with bark on the edges. Slabs can't be used like regular boards because the edges won"t be square, but you could use a nice one like this to do a carving on it's best side.
The sawcut on the back of this piece daylighted out about the serpants head on the right side, and you can actually see evidence of some old bark on the back of it there. Not the rough outer bark, but the inner membrane that carries the nutrients. Some of it has flaked off with age of course. I used to be a treeman, and I've probably cut up a thousand dead trees, so I know what the outside of a tree looks like when the dead bark falls off.
Anyhow, I can't see that there could ever have been more carving on top of this carving. They used all the board surface that was there to use.
So I've made my case the best I can with words, where handling the objegct and seeing it up close isn't an option. Believe or disbelieve as you will