I recently came into possession of 2 Belgian 20 francs dated 1935
They are not rare and pretty common. My first impression would be that they are fake (due to the bubbly damaged surface) but all the measuremens are fine (also did the specific gravity test). I also don't find any castmarks on the edges. These coins are pretty common so I wouldn't know why would anyone bother to counterfeit them. Those are .680 silver.
So if these are not fakes (not yet sure), I was wondering what could have damaged the surface like this.
Maybe some experts here could help me out.

