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Help me identify this

No.1875425 View ViewReplyOriginalReport
I found this stone disc today when I went fishing to a remote creek, does anyone have an idea what could it be?
As you can see it has circular ridges on one side, the other one is blank. Diameter is 19.3 cm, thickness is 1.5 cm..
It's too small to be a millstone and too smooth to be a grinding stone (plus there's no hole in the middle). It also doesn't strike me as a coaster or a base of some pot - why would you put so much work in such a simple item, making it out of stone.
I live in the Alps, there's a small village 2 km away from this creek, there are historical settlements, but way down the valley.
I'm perplexed by what this perfectly circular disc made out of stone was doing in the creek, perhaps someone can help me?

pic related it's the disc