Looks like you could use it to remove the crown cap from a beer bottle.
Seriously, I have no idea, but if it's there for a reason, SOMEBODY here will certainly know. I've seen photos of movement plates that had cutouts matching key heads, escape wheel bridges, etc. Maybe it's something like that.
I have small (4,5 cm), unmarked winding key in this shape (without the clock). The purpose of this "thing" seems to be the same. But this is not a hanger in this case, I think... Purpose? Adjusting something? The pendulum nut?
And only dwarves can use it for opening their small beers
This may be a tool for removing transport protection nuts or adjusting/mounting leveling feet for instance. A kind of a pin/hook wrench.